TN 24 
6 L 


rc. i ^ 



■ . . ; 4 1 i ' 

i)\‘. o\\ 


f 2 t V lt ‘ 
■' f i { K / > 




■ J •, • • ' ' * • i .* * v •" <■ * • <; < j ( * r • I;: t • r • / , . ir/;j. ; 

• v< • - • ■■ 1 lil ■ ‘J ,s'. . • /' ; 1 : y ‘ f 

: ; ■ • .v •. ‘ ■ . . - . ,! . *.I ! !ff •- ' i , 


;•< ft - * * :• • . • . ■ i - : ... - : . l *• >7. i i v -• % c » v < y . •, 

. ’ ; ' 11 r . r * .' ' ll' ‘ ' y » J • J ’ ■' f ; • , j « f i 

: <•!.' •'r ., ■ V■ ')’ •;■, y ;{,• JV:‘ / r!*1ifHi ” 1 

: ; f • oo if-ii-j?i•' f<:■;-:{‘- 

' i - i ' i !'«.*{ y ! ■ t'.-“ \ ' V - 5 ■ ' • £ * i S 

I •« a , v t . * . 4. i i r * ' i • \ i ,- \ **.>'* * * ' ' • * : < 1 ' * 1 /'*'■••• *' ■ :* 

• -(j t r ■<'r i / ■ f.> J/ /• j* .•*»•?*.? i ' 

i . .. , ' , r.l\\ ' t \ : ' ; . • , / { ' » .M ,0 . •. » t '• * -f / f ! • J i t-’i ♦ ? 


, / r i , ‘ ' , ' ' 1 , f '• , ’ t J' ' . , • , f * - ‘ , i £ • * » / 1 « • r \ f , ft V 

, < ' ("'J . « / '■ i ; ' y ' 1 ' / ' r ^ i 1 / ' 1 ii 1 J 1 . • j ( ;/'i / , / it <>f>M / • 

•• i.- I / ■ ’ i ‘ } • ■ ’ } S' • * \ i ' l - f < . !«».#«• 

; v: 

: : • .■' ]<!:i'l • (i = •;?! in-: .j -11 


1-M 


"V < «;‘ r-4‘ 
\:; / ’ 


• )*.(. c, <• Jfl'iUCK'. V.\ r \W. J - 

; v•* ',) ]>] ’■]: V./r.kjj• il '4 <*i'ii 1 J-'«!w 

mhmtti 


f.j. 11 1 


* «»i ‘ ^ 1 s 


< 4 ' 

i. v I 


i.i, j']i 


m, r.f- 

i'i :T.' !v 


in :; ; ih•;v V. 1 ■'I:Sil lier ? i 


. rsgiiptlHiii WM 
W- : r ■ i;\} itSii i i?;l ' 

) ‘ i ' s J» • * ";v,:)v. v: i i ; .> . *.-? v V i) ' . J J t ? V ? * i t- 5 


■•••:::■ : ;: • : ■; -•;'*, i : ■.* i•; i 1 \: ;|: •. : ? '. j, : , hi?Hi |’ j ■ j 4 u ;• •' •• i' 

i ,'hr (:J • ;•, - 5 '< • i • ,*'..hh ; j*{, t S i ; i } \ < s m5}i4 

i •' s;\ ' t v• i . • < t\: lh : .'^h * > < f S< '■>■* tS• i i 























s 
















* 

























\l/ 


By prof. f). lal^es. 

LIBRARY OF CONGRESS, 

ftMFWr* 

DEC 101901 

DIVISION OF DOCUMENTS. 

■ 
























































































GEOLOGY 


£LqL \3 

—OF— 


Colorado 



Deposits. 


u 


PROFESSOR AT 


STATE SCHOOL OF MINES, 

GOLDEN, COLORADO 



DENVER, COLO.: 
News Printing Company, 
i 888. 


















T H t . 

CcsU\s 

¥ 


0 


' 6-/330 





PREFACE. 


This treatise contains the substance of a series of elementary 
lectures delivered by the author to the students of the Colorado 
State School of Mines. It is published with a view of meeting 
some of the needs of the general public, of the ordinary miner, 
and of the unscientific many, rather than with any idea of offer¬ 
ing original matter for the discussion of the scientific few. The 
materials are derived partly from the writer’s own researches, and 
largely from the most reliable resources available to him, such as 
the standard text-books and the valuable published reports of 
the United States Geological Survey. The writer having for a 
time been connected with that survey, knows full well how high 
a value is to be attached to those reports, which must remain for 
all time as a standard of reference for those interested in the 
geological relations of the mines of Colorado. 

The first part of the work contains a rough sketch of general 
Geology, for the benefit of those who are not familiar with the 
terms used in this science. The sketch is further applied to the 
local geology of Colorado. The second part refers to the phe¬ 
nomena of veins and ores, and their surroundings, as illustrated 
by Colorado. 

The last part contains a brief account of some of the principal 
mining districts of Colorado, not as official reports of those dis¬ 
tricts, but as examples to illustrate the principles of the preceding 
parts. 




Part I. 


GENERAL GEOLOGY. 

To assist such of our readers as may not be very familiar 
with the science of Geology, in understanding such technical and 
geological terms as are unavoidable in this treatise, we offer a 
rough general outline of the earth’s history, applying it after¬ 
wards to a sketch of the geology of this particular region of 
Colorado. 

ORIGIN OF THE EARTH. 

The world was not “ spoken into existence ready made ” in 
the state we now find it. It has attained this condition through 
a multitude of gradual changes and revolutions which have 
taken millions of years to accomplish. The remote history of 
the earth is a matter of hypothesis. There are reasons for sup¬ 
posing that at one time its elements were in a gaseous condition, 
and that this planet was an incandescent luminous cloud revolv¬ 
ing through space, gradually consolidating into a molten ball, 
surrounded still by an atmosphere of gases, a condition perhaps 
not unlike the present one of the sun, whose interior is supposed 
to be passing into the molten state while its exterior consists of 
various incandescent gases arranged more or less according to 
their specific gravities. The spectroscope has detected the ele¬ 
ments of some of our earth metals and minerals in the sun in a 
state of vapor. 

Upon the cooling of the ball, a crust formed like that on 
molten iron, crumpled, by contraction due to cooling, into an 
uneven surface with slight elevations and depressions, and doubt¬ 
less broken through, here and there, by gre^t fissures and vol¬ 
canic craters, through which the molten flood beneath poured 
out in volumes. 

Upon such a surface the gaseous atmosphere gradually con¬ 
densing, descended as hot chemical rain, and filled the troughs 
of the crumpled surface with a hot chemical steamy ocean. 


VI 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Whatever land of primitive lava arose above this ocean was bat¬ 
tered by the waves, reduced to sediment, and deposited as the 
first stratum in the bed of that primaeval ocean, the eruptions 
from below the thin crust also contributing largely to the same 
material. 

ARCHAEAN AGE. 

Thus we may suppose was formed the first stratified rock of 
the world which we have an opportunity of actually seeing, that 
is granite with its varieties of gneiss, schist, syenite, etc., and as 
this is the “ beginning ” age, within human observation, we call 
it Archczan , the Greek for beginning. It is, however, evident that 
these granite rocks which form the axes of our mountains are 
not the very first, because their stratification and composition 
show them to have been derived from the breaking up of still 
earlier rocks, which latter may have been the original cooling 
crust. 

SILURIAN AGE. 

Cooling and contraction still progressing on the globe, fresh 
and greater wrinkles were caused on the surface of its crust, and 
some of this granite sea-bottom was crumpled up till the crum¬ 
ples arose above the surface of the ocean as low islands or reefs. 

The ocean had now cooled sufficiently to support marine life 
and so along these granite islands, corals formed reefs,, shell-fish 
swarmed, and sea-weeds grew. Sandstones formed by the 
waves from the material of the granite were laid down as shore 
line strata, often mixed with shells, and in deeper water, coral 
limestone was in progress, as it is at the present day. 

Hence in Colorado we find the upheaved crumpled granite of 
the old island, with its sandstone or quartzite beach, with fossil 
shells, and upon that, limestone, with fossil corals still visible. 
We call this the Silurian age, and the fossil shells and corals 
Silurian fossils, because they are peculiar to that age and quite 
unlike the shells and corals of the present time. 

North America, at the beginning of this period, was barely 
outlined by a few granite islands. One formed the site of part 
of modern Canada, one or two reefs or islands marked the site of 
the eastern ranges of mountains, and a few parallel granite islands 


GEOLOGY OF COLORADO ORE DEPOSITS. 


vn 


rudely outlined the site of the principal uplifts or future great 
ranges of the western Cordilleras. All else was Silurian ocean 
and that ocean was depositing its Silurian coral limestones and 
sandstones against these few granite islands, destined in time to 
grow into mountain ranges and to become the backbones of the 
continent. 

DEVONIAN AGE. 

Upon the Silurian followed the Devonian, an age character¬ 
ized by extraordinary fishes. But as it is not generally repre¬ 
sented, for some reason, in these mountains, we pass on to the 
Carboniferous. These ages we are speaking of are separated 
from one another by decided and characteristic changes in the 
fossil animal and vegetable life existing between one age and the 
other, and in various parts of the world these changes of life are 
marked also by great geological revolutions such as the eleva¬ 
tion of mountain ranges, or great continental masses. 

In America these oscillations between sea and land seem to 
have been less than in Europe, and we find a general uniform rise 
of the continent from the ocean and an orderly succession of 
strata lying against the flanks of the ever rising granite nucleus 
of both mountains and continent. 

CARBONIFEROUS AGE. 

In the Eastern States, as the continent gradually began to 
rise from the waters, and to the granite islands had been added 
a Silurian shore, and to that a Devonian shore, a kind of trough 
seems to have been formed between the middle and eastern part 
of America, which was occupied by very low, marshy land or 
marshy islands barely above the sea level. Upon these low-lying 
lands grew a dense vegetation unlike any of the present day, but 
resembling somewhat our tree ferns of the Southern States. 
This low land was subject to freshets from surrounding higher 
regions which periodically deluged the swamps and its vegeta¬ 
tion with river and flood deposits of pebbles and sand, under 
pressure of which the vegetation was gradually turned into coal. 
Successive coal beds were formed by successive growths of vege¬ 
tation between the intervals of periodic inundations, subsidence 


Vlll 


GEOLOGY OF COLORADO ORE DEPOSITS. 


and upheaval, for these low lands also appear to have occasionally 
subsided under and been lifted up again above the sea. Finally 
by a grand revolution which closed the Carboniferous age in 
America, the coal swamps and strata were crumpled up into the 
great Appalachian range of mountains. In the Rocky Moun¬ 
tain area the marine condition seems during this time to have 
been the prevalent one, for we find a predominance of marine 
limestones and sandstones with very slight indications of coal or 
land plants. 

In Colorado the Carboniferous limestones with fossil corals 
and shells in them, lie directly upon the Silurian deposits. Upon 
the limestones come thick b£ds of sandstone and shale, which 
latter may mark a land and fresh water state of things, resem¬ 
bling that in the East, for we not only find traces of coal in the 
South Park range and at Aspen, but also a few fossils of those 
strange Carboniferous tree ferns and horsetail rushes or equiseta. 
It was not, however, the great coal-bearing age in Colorado as it 
was in the Eastern States and throughout the world generally. 
Our great Colorado coal-bearing era is a much more recent one, 
viz. : the Cretaceous. 

The Carboniferous, however, contains in its limestones much 
of our mineral wealth, such as the silver-bearing lead deposits of 
Leadville, Aspen and other regions. 

The Silurian, Devonian and Carboniferous ages have been 
grouped together into one great division called the Paleozoic time 
or “old life” of the earth. By Paleozoic rocks then, we mean the 
rocks of one of these ages, or rather (in Colorado), only the 
Silurian and Carboniferous, as the Devonian is missing. 

TRIASSIC AND JURASSIC PERIODS. 

V 

After the Carboniferous followed the Triassic and Jurassic 
periods, whose rocks in Colorado are marked by their prevailing 
red color and consist of heavy bedded red conglomerate sand¬ 
stones such as may be seen in the gateway of the “Garden of the 
Gods” at Manitou. In South Boulder canon, in Platte canon, 
at Morrison, in Bear Creek canon, and almost in every canon in 
the foothills, in the Gunnison region and in the neighborhood of 
Aspen, the same red rocks are very prominent. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


ix 


In South Park also they are observed on the flanks of Silver- 
heels mountain, and in the neighborhood of Fairplay and the 
salt works. Besides the sandstones there are thin beds of lime¬ 
stone and variegated clays. The conglomerate sandstones and 
limestone show no evidence of fossil life, but the variegated clays 
in the upper portion of the Jurassic have yielded at Morrison and 
near Canon City some remarkable remains of gigantic lizards 
called Dinosaurs. It is probable from the presence of salt and 
gypsum and the redness of the rocks that these red beds were 
laid down in land-locked salt seas or salt lakes, shunned by ani¬ 
mal life. 

The upper portions, however, show evidence of low, marshy 
land and fresh water, by the presence in them of turtles, croco¬ 
diles, fresh water shells and Dinosaurs or land lizards. These 
upper deposits may be of estuarine origin. 

CRETACEOUS PERIOD. 

t 

After this followed the Cretaceous period, a very thick forma¬ 
tion numbering several thousands of feet in Colorado, consisting 
in its lower and middle portion of sandstones, white limestones 
and very thick beds of drab clays. These are mostly marine, as 
shown by the sea shells found in them. The exception is the 
Dakota group or Cretaceous No. I, at the base of the Cretaceous, 
which, along the foothills forms a prominent “hogback” of white 
sandstone. In this are fossil impressions of leaves of trees not 
unlike (but not identical with) those of the present time. The 
middle or Colorado group of the Cretaceous, consists of lime¬ 
stones and very thick beds of clay. Both limestones and clay 
contain quantities of fossil marine shells such as the Nautilus, 
Ammonite and Inoceramus, the two former resembling a ram’s 
horn or a snake coiled up, the latter not unlike a clam shell. 

LARAMIE COAL GROUP. 

The upper portion of the Cretaceous is called the Laramie 
group, and contains our vast and valuable coal fields. Associ¬ 
ated with these rocks we find also fossil leaves of semi-tropical 
vegetation, such as the palm, fig, magnolia,’ maple, etc. This 
Laramie group marks an important era in our Rocky Mountains, 


GEOLOGY OF COLORADO ORE DEPOSITS. 


x 

for it shows the beginning of the great Rocky Mountain revolu¬ 
tion by which the granite islands against which all these marine 
sediments had been forming, were elevated 10,000 feet or more 
into mountainous masses, dragging up with them portions of the 
sea bottom and exposing it as land surface, draining off the shal¬ 
low Cretaceous sea which had hitherto divided the eastern half of 
the American continent from the western, and bringing on a land 
and continental condition, which was completed in the following 
Tertiary age and has continued to the present. The Triassic, 
Jurassic and Cretaceous are grouped into the Mesozoic, or “mid¬ 
dle life” age. 

TERTIARY AGE. 

The Tertiary age which followed is not so well represented 
in Colorado as the Cretaceous. It seems to mark an era of com¬ 
parative rest in mountain elevation, for its sandstones and shales 
lie nearly horizontally on top of the upturned Cretaceous and 
other beds. These beds appear to have been formed by wide 
lakes of fresh water surrounded by tropical foliage. They may 
be seen capping the Divide between Denver and Colorado 
Springs, forming the singular “ mesa” or table land country, from 
Castle Rock and Sedalia down to Monument Park and Austin’s 
bluffs, east of Colorado Springs. From the singular forms cut 
out of these sandstones by water in Monument Park we call 
them the “ Monument Creek Group.” A similar formation is found 
between Colorado Springs and South Park at Florissant, right in 
the heart of the front range. It is a small Tertiary lake deposit 
remarkable for its fossil insects, petrified trees and leaves. 

In the neighboring territory of Wyoming the Tertiary lake 
beds form the great Green River region with its fossil mammals, 
fishes, leaves and insects. 

The Tertiary was the world’s tropical summer, a period ot 
beautiful lakes and tropical vegetation, but in certain regions it 
was disturbed by gigantic revolutions which upheaved the Him¬ 
alayas, the Alps, and other great mountain ranges. Such revo¬ 
lutions as occurred in our cordillera system were marked by 
frightful ebullitions of lava issuing from cracks and fissures, del¬ 
uging Idaho, Nevada and part of Oregon and Washington Terri¬ 
tory. Remnants of the Basaltic flood are found in Colorado, 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xi 


capping our coal table lands along the foothills, particularly 
toward the southern portion of the State and down into New 
Mexico. 

GLACIAL EPOCH. 

The Tertiary Summer was closed by the world’s great Winter, 
from causes which we will not here discuss. The ice from the 
North Pole extended its domain nearly to the Equator. All the 
northern temperate regions of the world were ice-sheeted, .and 
the sheet extended itself as by long fingers down the now highly 
developed mountains, filling every ravine with a glacier. It was 
the Glacial Epoch. In Colorado these glaciers occupied every 
incipient canon previously begun by streams or by folds in the 
strata. By their downward destructive movement they widened 
and deepened the canons, gouged out the mountains, and exposed 
the fissure veins, and did the first great mining on a gigantic 
scale in Colorado. The debris they carried on their backs, 
dumping it at the outlet of the canons, and when the tempera¬ 
ture rose and the glaciers melted, all the long lines of traveling 
boulders scattered upon their backs were left as banks, or “ mor¬ 
aines,” or “placer” grounds along the sides of our streams and 
canons, often a thousand feet above the present river bed, mark¬ 
ing the height and thickness the glaciers once attained. 

QUATERNARY AGE. 

So were our canons largely formed and so did our gold plac¬ 
ers originate. After the Glacial Epoch a warmer period set in, 
which we call the Quarternary. The ice melted, vast bodies of 
fresh water were distributed in wide streams and great lakes over 
this hemisphere. The rough morainal dumps of the glaciers 
were sorted or “modified” by water, rolled into pebbles and sand, 
re-distributed along the banks of streams, and carried out into 
the beds of the lakes. In these pebbles and sand was much of 
the precious metal, robbed from the veins. The gold by its 
insolubility in water remains to this day in our placer beds and 
“wash,” and is collected by sluice or hydraulic mining. 

Still the agencies of nature are going on as of yore. Conti¬ 
nents are gradually rising or sinking. Mountains are being imper¬ 
ceptibly elevated, water is still sculpturing them with canons, 


Xll 


GEOLOGY OF COLORADO ORE DEPOSITS. 


rivers are carrying down fragments robbed from the land and 
depositing them in the ocean to form strata for future continents. 
The fires of the earth are not yet dead, for volcanoes still vomit 
lava. The earth is still continuing to lose heat, its crust is still 
contracting and wrinkling itself upward, for we find modern sea 
beaches raised high on our cliffs. Shocks of earthquakes from 
time to time prove that motion of some kind is going on beneath 
us, and doubtless our mountains are still rising imperceptibly, 
as they appear to have done in the countless ages of the past, 
and slowly elevating and tilting strata that since the Tertiary 
period have lain apparently undisturbed. I say apparently, for 
even the Tertiary beds show everywhere a dip of 2 to 5, and even 
10 degrees, proving that the mountains have risen that much 
since these beds were deposited, and that they are probably still 
rising. 

For local illustration of the foregoing, we may descend by the 
course of any of our streams down its canon in the mountains 
till it debouches on the prairie. For forty or fifty miles the pro¬ 
found canon is through solid granite or gneiss.. The composi¬ 
tion of the granite is crystalline. It shows indistinct signs of 
once having been stratified. Its strata, moreover, shows evidence 
of intense folding and crumpling, as if by lateral, tangential press¬ 
ure such as would be caused by contraction. 

This is the Archaean granite that first lay as horizontal sandy 
strata in the bottom of that earliest hot and chemical ocean where 
it was crystallized. 

It was then crumpled up into the Colorado-Front-Range 
island above the Silurian sea, and formed the shore line for ages 
of seas depositing horizontally the different strata of the Paleozoic 
and Mesozoic eras. 

At the close of the Cretaceous or Mesozoic era it was further 
crumpled up from an island reef to a mountain range 10,000 to 
14,000 feet above the sea level. This movement added new 
crumples and foldings to its already puckered strata. The heat 
of friction partially melted some of its material, which filled 
fissures caused by the uplift; hence we find dykes of feldspar or 
“eruptive granite,” here and there. Heat also seems to have 
rendered the whole mass of strata more or less plastic. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


.mi 


As we emerge from the granite on to the foothils and prairie 
we encounter, as we go down the creek, the upturned beds of the 
various periods we have described, resting at a steep angle against 
the flank of the granite, in their geological sequence and order. 

First are beds of sandstone and limestone, with fossils such 
as trilobites, Crustacea, and spirifer shells, representing the Silu¬ 
rian. Next, variegated beds, with limestone at the base and 
coarser sandstone and shale near the top. In the latter some 
traces of coal and coal plants mark its Carboniferous character. 

Upon this follows a great thickness of coarse conglomerate 
sandstones, variegated clays, and some limestone, all of a general 
red or variegated character. 

These represent the Triassic and Jurassic periods, and in the 
clays of the latter, Dinosaur bones are found at Morrison and 
Canon. 

A prominent “hogback” or ridge next appears on top of the 
Jurassic clays. It is formed of hard white or gray sandstone, on 
which are some leaf impressions. It is the first group or base of 
the great Cretaceous period, and is called the Dakota group, or 
Cretaceous No. I. 

After we pass this “hogback” we generally find a flat, grassy 
valley, underlaid by soft, dark shales and clays, full of marine 
Cretaceous shells, with one or two belts of limestone, also full of 
large shells. Evidently this is the bottom of the Cretaceous sea, 
and represents the Colorado group of the Cretaceous. 

The next uplifted sandstone we meet with shows sea-weed 
fossils in it, and a little higher up, land plants, and then two or 
three coal beds. This is the uppermost group of the Cretaceous, 
known as the Laramie coal series. 

A few scattered table mountains of horizontal strata, with fossil 
leaves, may locally be met with, as on the Divide, which repre¬ 
sent our Tertiary period. And lastly, on top of all these strata, 
both horizontal and upturned, we find scattered over “hogback” 
and prairie alike, the drift or “wash” of pebbles of all kinds, dis¬ 
tributed by the glaciers and floods of the Quaternary. 

Thus since we left the granite at the outlet of the canon we 
have passed through and examined not less than 10,000 feet in 
thickness of the crust of the earth, comprising all the periods 


XIV 


GEOLOGY OF COLORADO ORE DEPOSITS. 


from Archaean to Tertiary and Recent We have found the 
strata of the several periods and epochs lying upturned against 
one another, like leaning rows of books, till we come to the top 
of the uppermost Cretaceous or Laramie coal, and then the Ter¬ 
tiary lies flat. We conclude, then, that the strata of all these 
periods and epochs were accumulated and lay undisturbed and 
horizontal until the close of the Cretaceous, when the granite 
island was elevated into a mountain. Since then elevation has 
been quiet or very slow. 

Such are the general geological features of the eastern slope 
of these mountains and their attendant foothills. If we penetrate 
into the heart of the mountains, to the region of South Park, and 
thence across to the western slope, we shall find much the same 
features repeated, and with much the same geological history. 

In South Park, on the western side of the “Front Range," 
we find its basin underlaid by the same rocks of the same geolog¬ 
ical periods, from Silurian to Tertiary and Quaternary, as we find 
on the Eastern foothills; the same Post-Cretaceous uplifting of 
Paleozoic and Mesozoic rocks upon the flanks, even to the top 
of the granite mountains, and the same pause in elevation marked 
by the horizontal Tertiary and Quaternary beds. Also the same 
evidences, only much more distinct, of the former presence of 
glaciers, by morainal placer beds, and by the U shape of the 
canons, with marks of the action of Quaternary floods, following 
the melting of the glaciers, in the distribution of pebbles and sand 
all over the surface of the park. In the heart of our mountains, 
however, we find that the folding of the rocks was more violent. 
Eruptions of lava incident on such movements were more abun¬ 
dant, and evidences of former heat are more apparent than nearer 
the foothills. This is shown in the fact that the unaltered Silu¬ 
rian and Carboniferous sandstones we met on the foothills, are 
here changed by heat into hard white vitreous quartzites. The 
limestones in many places pass into marble, shales and clays into 
slates, and in the Gunnison region the Laramie coal is turned 
into Anthracite. 

The Colorado “PYont Range" and the Sawatch Range each 
being surrounded by the same set of marine and other strata, 
shows them to have had the same history; first as horizontal 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xv 


granite strata, next lifted into granite islands in the Silurian 
ocean, and lastly, at the close of the Cretaceous, elevated into 
mountain chains, carrying up with them the various beds so long 
accumulating in the seas by which they were surrounded. 

With these preliminary explanations, the following epitome of 
a sketch of the general geological history of these mountains, 
from the accurate observations of the U. S. Geological survey, 
will be intelligible. 

At the close of the Archaean era, when the earth was cov¬ 
ered by a Silurian ocean, a large area covering most of what is 
now the Colorado, or “ Front Range,” formed a large rocky 
granite island, with a number of smaller islands lying to the 
West of it, the most important of which now forms the 
Sawatch Range from which it was more or less completely sepa¬ 
rated by the sea waters occupying the present depressions of the 
North, South and Middle Parks. During the whole of the 
Silurian, Carboniferous, Triassic, Jurassic, and Cretaceous periods, 
i. e., the Paleozoic and Mesozoic times, a continuous deposit of 
sediment went on in the seas surrounding these islands, of 
materials such as sand and pebbles washed from these granite 
islands, and also of organic limestone derived from corals and 
shells. 

No great disturbances took place throughout this long period, 
hence all the different strata with their various fossils lay for the 
most part conformably and horizontally one on top of the other 
in successive order. 

Toward the close of the Cretaceous period, that is at the end 
of the Mesozoic, at the time of the formation of the coal beds, the 
seas became shallower, owing to a general elevation of land, and 
considerable portions of the outlying area were partially enclosed. 
During this time and possibly earlier, immense masses of eruptive 
igneous rock were forced up through the already deposited sedi¬ 
ments still lying horizontally beneath the waters. 

Unlike the lava flows of modern days, these molten masses 
did not spread out on the surface of the rocks, but congealed 
before they reached that surface either in large masses, in dykes 
or in sheets spread out between the strata. These phenomena are 
well exhibited in the Leadville, Gunnison and Aspen districts. 


XVI 


GEOLOGY OF COLORADO ORE DEPOSITS. 


We do not know how long before the Cretaceous the erup¬ 
tions of these igneous rocks commenced, but they certainly con¬ 
tinued to the close of that period, for we find them traversing 
rocks of that age—as at Crested Butte, Irwin, and Gothic, in the 
Gunnison region. 

Some time after the close of the Cretaceous a general upward 
movement took place in the Rocky Mountains, by which the 
existing mountain ranges or islands were crushed together, 
broken and elevated, and considerable areas of the adjoining sea 
bed were lifted above its surface. 

In the general continental elevation which followed during 
the Tertiary period, fresh water lakes or enclosed seas were 
formed, in which, by the washing away of the newly made land 
areas, considerable sediments were deposited, such as the strata 
on the divide and at Florissant, and the table land country 
generally. 

D uring this Tertiary era and after it, eruptions of lava also 
occurred, generally following the lines of earlier and older erup¬ 
tions, but unlike the latter, spreading out on the actual surface of 
the land, and in some cases beneath the sea, as for example, the 
basaltic cap of “ Table Mountain,” (Golden), Fisher’s Peak, near 
Trinidad, and the rhyolite capping of the mesas of the Divide. 

While the general form of the mountain area was sketched 
out and determined in the earliest geological times it is only 
since the Tertiary, and in a great measure by erosion after the 
Glacial epoch, that the present sculpturing of the mountain forms 
with their ravines and canons has taken place. 

GEOLOGICAL AGE OF MINERAL DEPOSITS. 

At what period the different mineral deposits of Colorado 
were formed cannot be definitely stated. 

The gold deposits of Gilpin County may have been during or 
after the Archaean age, since they occur in Archman rocks, but 
as in the immediate vicinity of these deposits there are no later 
rocks to limit their exact age, they may have been before the 
Silurian or very much later. 

The silver-lead deposits of Leadville were certainly formed 
after the Carboniferous, and before the mountain upheaval at the 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xvn 


close of the Cretaceous, because in the first place, they penetrate 
Carboniferous rocks, and in the second place, the fissures, faults, 
etc., formed at the time of the Cretaceous uplift, cut through and 
fault these deposits. 

Those of the Gunnison region are later than the Cretaceous, 
because they occur in fissure veins, cutting through the Creta¬ 
ceous rocks, even through the Laramie coal beds. 

Those of Custer and San Juan were probably formed in the 
Tertiary, because they traverse basalts and other lavas of “ pre¬ 
sumably Tertiary age.” 

CONNECTION BETWEEN MINERAL BELTS AND MOUNTAIN UPLIFTS. 

In the greater Cordillera system, of which our Rockies are a 
part, there appears to be a definite connection between min¬ 
eral belts and well-known lines and times of uplift. There are 
several well defined, more or less parellel mineral belts in the 
Cordillera system. 

There is one at the foot of the Wahsatch Mountains, repre¬ 
sented by the Utah mines, which lie in the foothills of this range 
with a definite relation to the main line of crests. 

The gold and copper belts of California stand in a similar 
relation to the Sierra Nevada. 

The quicksilver and cinnabar belt of California is a belt par 
allel to the Coast Range. 

The Arizona belt lies in a northwest and southeast direction 
diagonally across the country. 

The mining districts of Nevada cannot be so easily grouped. 

Now these four distinct belts are connected with four great 
mountain building changes and uplifts, which the region West of 
the Rockies has undergone. 

The last of these was after the Miocene Tertiary, resulting in 
the uplift of the Coast Range, together with part of Oregon and 
Washington Territory. The disturbing force was most powerful 
North and South of San Francisco, and there lies the cinnabar 
belt. An upheaval soon after the Cretaceous, raised the whole 
western central portion of the continent, now occupied by the 
complicated system of the Rocky Mountains. 


seinu 


GEOLOGY OF COLORADO ORE DEPOSITS. 


The Wahsatch Range is the western edge of this uplift, and 
the dislocation took place on an old fault, coincident with the 
present western foot of that range, and here, as we have said, lie 
the mines of Utah. 

The Sierra Nevada and ranges of the great basin were raised 
by an uplift at the close of the Jurassic. 

The line of most intense disturbance coincided with the 
Sierra and the greatest dislocation occurred along its . western 
foot in what is marked by the gold belt. 

The earliest disturbance in the far West was that which raised 
the Paleozoic strata of Eastern Nevada, Western Utah and part of 
Arizona above the surface of the ancient sea, extending over part 
of the plateau and Colorado river region, past Prescott and on 
to Tombstone, Arizona. The main Arizona belt nearly coincides 
with the borders of the Paleozoic uplift. 

The age of mountain uplifts we judge by the strata involved 
in that uplift, thus we know that the great uplift of the Rockies 
occurred at the close of the Cretaceous, because both the Creta¬ 
ceous rocks and all those of previous ages are uptilted with the * 
mountain while the Tertiary rocks are not. 

These uplifts are not the immediate cause of mineral belts, 
but rather of Assuring and faulting. 

The uplift in Nevada, at the close of the Carboniferous, was 
gentle without much crumpling of rocks, hence the number of 
ore deposits is not so great along its edge; those we do find, how¬ 
ever, such as at Battle Mountain and Cerro Gordo, are on the 
edge of the uplift, and are rudiments of the belt, better defined 
in Arizona, where the uplift was more violent. 

Thus there appears to be a relation between ore belts and 
lines of uplift, from which we may infer that the great Post-Cre¬ 
taceous uplift of our> Rockies determined some of the lines of 
our ore belts, and we shall find the fine and time of uplift of 
some of the minor ranges in Colorado more or less coincident 
with the mineral belts found in them. 

The uplift of a great range is not along a single line of dislo¬ 
cation, but such movements are accompanied by a great number 
of parallel faults, and parallel sets of fissures, with stringers run¬ 
ning off into the surrounding region. A large belt of country is 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xix 


fractured by innumerable rents in many directions, which may 
afterwards be filled by veins, and constitute a mining district. 

PRINCIPAL ORE-BEARING ROCKS. 

In Colorado gold is found in the Archaean granites, gneisses, 
and schists, and also in the eruptive porphyries of Mesozoic or 
older age, and in the placer beds derived from these rocks. 

In sedimentary formations, it is rare in limestone but occurs in 
.quartzites of Silurian or Carboniferous age. 

Silver is found in the Archaen and in the porphyries, and 
occurs especially in the limestones of Carboniferous age, as also 
in those of the Silurian, and locally in some of the Cretaceous 
Rocks. 

The most important ore deposits are found where igneous 
eruptive rocks abound, especially in rocks of older date, as por¬ 
phyries, rather than in the newer Tertiary volcanic rocks, such as 
basalts. 

The “hogbacks” along the foothills consisting of upheaved 
limestones, clays, shales and sandstones yield no precious metals, 
and it is only when the mountains have been penetrated to 
their core that precious minerals are found productively. 

The mineral deposits of Boulder are the nearest to the “hog¬ 
backs” and plains in this respect, some of the mineral belts being 
only two miles distant from the foothills. 

Though the sedimentaries forming the “hogbacks,” foothills 
and plains are not found productive, yet in the heart of the 
mountains where foldings, crumplings and volcanic eruptions 
have occurred, we find the same rocks quite prolific in minerals, 
notably in the Gunnison, South Park, Aspen and Leadville 
districts. 






Part II. 

COLORADO ORES AND THEIR MODES OF OCCURRENCE. 

Metals occurring in a nearly pure state are called “native.” 
Native gold in Colorado is sometimes visible to the eye in the 
form of little hairs or wires, or as minute grains or thin flakes 
upon a piece of ore, quartz or rusty material, but very rarely as 
nuggets in a vein. Nuggets, together with fine flakes of gold, 
are found in the gold placers. Native silver also occurs in little 
bunches of wires, and is sometimes called “ wire silver.” It is 
also visible as flakes or strings in vein matter. Native copper is 
occasionally found in thin plates in crevices of the rock. Metals 
united with non-metallic substances form “ores” proper. Most of 
our precious metals in Colorado are found in this condition. 
Metals unite with non-metallic bodies, as sulphur or chlorine, 
forming sulphides, (ex. gr. iron pyrites or galena,) or chlorides, 
such as horn silver; with oxygen, forming oxides, as oxide of 
iron or of manganese; with acids and salts producing carbonates, 
sulphates, etc., for example: carbonate of lead, (cerussite,) sul¬ 
phate of lead, (anglesite,) carbonate of copper, (malachite,) car¬ 
bonate of iron, (siderite.) 

Any material containing a workable proportion of a metal is 
commonly called an “ore.” For example, quartz carrying gold- 
bearing pyrites, or limestone containing silver-bearing galena. 
Ores are found in surface deposits, such as gold in placers, dis¬ 
seminated through igneous eruptive rocks, such as porphyries, 
also through sedimentary rocks, such as limestones, between 
stratified formations, such as between layers of quartzite, lime¬ 
stone, or gneiss, and in veins of different kinds, traversing all 
kinds of rocks. 

The non-metalliferous and earthy minerals associated with 
the ore, such as quartz, calcspar, baryta, are called “ matrix,” 
“veinstone,” or “gangue.” 


XXII 


GEOLOGY OF COLORADO ORE DEPOSITS. 


GEOLOGICAL OCCURRENCE. 

Metals occur in Colorado in rocks of every geological age, 
principally in the mountainous districts and in the older rocks, 
especially at the junction of igneous eruptive rocks with sediment¬ 
ary rocks. 

The Archaean age is represented by the veins of Georgetown, 
Central and Boulder. 

The Silurian and Carboniferous by the ore deposits of Lead- 
ville and South Park. 

The Triassic by some copper stains in various localities, and 
a few lead deposits. 

The Cretaceous by the veins of Crested Butte and Gunnison, 
and by iron-ore deposits. 

The Tertiary by the veins in the Tertiary eruptive rocks of 
the San Juan region. 

The Quaternary by the various placer deposits. 

The Archaean, Silurian and Carboniferous, i. e ., the older rocks, 
are the great ore producers. 

The plains and foothills yield no precious metals, only a few 
base ores such as iron. The precious metals are mostly confined 
to the heart of the mountains, and diminish in occurrence as we 
recede from it. 

The ore deposits at Leadville occur at the junction of eruptive 
quartz porphyry with limestone. Those of Georgetown, Central 
and Boulder are also frequently found at the contact between 
porphyry and gneiss. 

Metals also occur in sedimentary rocks which have been pen¬ 
etrated by dykes of eruptive rock, or have been exposed to 
great metamorphism. The Gunnison region around Crested 
Butte, Gothic and Irwin is a good example. 

The marine Cretaceous sandstones and limestones and the 
Laramie Cretaceous coal strata have there been locally riddled by 
dykes and volcanic masses, which, besides throwing the region 
into strange contortions, have also, by their heat, metamorphosed 
it, changing limestone into marble, sandstone into quartzite, 
shales into slates, and coal into anthracite. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xx in 


In this region veins of silver, lead and other ores are found, 
showing the striking connection between heat and its attendant 
metamorphism with vein occurrence. 

ORES FAVORING PARTICULAR GEOLOGICAL HORIZONS. 

Ores seem to favor some particular geological horizon, not 
because that particular geological age or horizon was one espe¬ 
cially productive of mineral at the time of its formation, but rather 
that the rocks of that age happen by peculiar circumstances to be 
better adapted for receiving mineral solutions than those of some 
other ages. Tlius the “blue limestone” of the Lower Carbonif¬ 
erous throughout our mountains has been particularly productive, 
not because it belonged to the Lower Carboniferous age so much 
as because it is locally penetrated by eruptive rocks, the limestone 
itself being favorable for receiving mineral deposits. Where the 
eruptive rocks do not occur the Carboniferous limestone is com¬ 
monly as barren as other limestones generally are in Colorado. 


GOLD PLACERS. 

Gold is found in surface deposits of sand and pebbles resulting 
from the breaking up of older rocks by glaciers, and the distribu¬ 
tion of the detritus by rivers, in what we call “placer ground.” 
Among the pebbles and sand, gold is found in flakes and nuggets. 
The gold is derived partly from broken up gold-quartz veins in 
these older rocks or from the disintegration of the constituent 
minerals composing the mass of the rocks themselves, which 
contain minute particles of gold in their elements, particularly the 
igneous or metamorphic rocks, such as porphyries and granites. 
These placer deposits are of recent origin, dating from the 
Tertiary or Glacial Epochs to the present, and are to be found in 
nearly every mountain ravine in Colorado, or on the banks of our 
principal streams. 

Those at Alma, (South Park,) along the Arkansas valley, and 
at San Miguel in the San Juan country may be cited as examples. 

Platinum and tin are found under similar conditions, but these 
metals do not occur in Colorado. 


XXIV 


GEOLOGY OF COLORADO ORE DEPOSITS. 


nature’s milling process. 

Gold quartz from a vein requires its gold to be artificially 
separated, first by crushing and next by water, the heavy metal 
sinking to the bottom, the lighter veinstone being carried off by 
water. In the case of placers which consist of boulders, pebbles 
and sand on the banks of streams or sides of canons, represent¬ 
ing specimens of the varieties of rock over a large area; nature 
has already performed this process on a grand scale. The glaciers 
have mined and torn off the gold-bearing quartz of the veins 
together with cubic miles of rocky material, also holding minute 
portions of gold disseminated through it. The streams have 
reduced this to pebbles and sand, have mechanically separated the 
gold, and also probably by the action of acids chemically set it 
free from other minerals in which it was contained. By its gravity 
it sinks to the bottom and is found in greatest quantities next to 
the “bed rock’’ in cracks and crannies. The miner, after all this 
crushing, concentrating and jigging by nature, has only to 
re-wash and sift the debris and collect the water-worn flakes and 
fragments of free gold with or without the assistance of quick¬ 
silver. 

CALIFORNIA PLACERS. 

The vast placers of California lie for many square miles on the 
Pacific slope of the Sierra Nevada. They are called “blue 
gravels,’’ and were brought from the range by glaciers and dis¬ 
tributed over the more level country by ancient rivers and by lake¬ 
like expansions of such streams. The fossil remains of elephants 
and plants in them show them to date from the Tertiary or 
Quarternary age. Afterwards these river deposits were covered 
by volcanic ashes issuing from eruptions in the Sierras and finally 
by streams of lava from the same source. This hard molten crust 
upwards of one hundred feet thick, protected the underlying gravels 
from being washed away. Chemical changes in these gravels 
have silicified and changed to opal the once water-logged tree 
trunks brought down in these gravels. Some have been changed 
into lignite coal before silicification, part of the wood resembling 
jet, and part opal. The gravels are also cemented by silica. 


XXV 


GEOLOGY OF COLORADO ORE DEPOSITS. 

Stone implements have been found under ioo fe^t of lava in the 
gravels, showing! man’s existence at the time the gravels were 
deposited. There are also several uncovered placers worked by 
hydraulics. Rivers flowing through the gold belt of California 
have acted as natural sluices of which the miner’s sluice is a 
diminutive copy, the upturned slates acting as “riffles” to catch 
and retain the gold. The Australian placers are very similar to 
those of California. 

COLORADO PLACERS. 

The placers of Colorado have the same history, with the 
exception of the lava capping. The glaciers of the Glacial epoch 
and the floods resulting from their melting in the Quaternary 
epoch have been the main distributers of our placers. The gold 
is doubtless in part derived from gold-bearing quartz veins and 
gold-bearing rocks higher upon the mountains than the placer 
beds. Thus the placers of Clear Creek derive their gold from 
the veins and gold ore-bearing region around Central, Idaho 
Springs and Georgetown. The placers at Alma and Fairplay, 
South Park, from the veins and eruptive rocks near Mt. Lincoln 
and Montgomery, and the park ranges. The Tarryall placers in 
the basin of South Park from the auriferous deposits lately dis¬ 
covered in the eruptive rocks in the mountains above Brecken- 
ridge. The rich placers in “California gulch,” now occupied by 
Leadville, and those widely distributed through the broad valley 
of the Arkansas, may have derived their gold largely from dis¬ 
seminations in the metamorphic granite and particularly in the 
eruptive porphyries, as gold veins have not so far been discovered 
in great abundance in that characteristically lead and silver bear¬ 
ing district. 

COLORADO PLACERS DUE TO GLACIERS. 

It is noteworthy that each of these localities shows unmis¬ 
takable signs of former great glaciation. Clear Creek had its 
great glacier descending from Georgetown and joined at the forks 
of the creek by one coming down from Central. The whole val¬ 
ley in which Alma and Fairplay are located shows similar signs 
of a great glacier descending from back of Mt. Lincoln and 
receiving tributary glaciers from Mosquito, Buckskin and other 


XXVI 


GEOLOGY OF COLORADO ORE DEPOSITS. 


canons. The basin of South Park was occupied by a glacial 
lake into which glaciers descended from the mountains around 
Breckenridge. The Arkansas valley was filled by a prodigious 
glacier receiving innumerable tributary glaciers from the canons 
of the Sawatch and from the slopes of the Mosquito Range. 

Upon the moderation of temperature and consequent melting 
of these bodies of ice the Arkansas valley was occupied by a 
broad river, and “lake-like enlargements” of the same, which dis¬ 
tributed the placer drift and gravel in banks and terraces over that 
area. That gold may be derived from the breaking up of igne¬ 
ous rocks seems probable from the “ black sands” of the Cali¬ 
fornia sea-beaches which consist of titanic iron derived from the 
breaking up of the eruptive rocks of that volcanic region. 
These sands carry small nuggets and fine gold dust, the latter 
often too fine to save by present processes. 

TIN. 

Platinum and tin are found in other countries, but not in Col¬ 
orado. Specimens of “stream tin” in dark brown, round nodules 
of the variety called “wood tin” showing a banded, jasper-like 
structure are found in the drift material in Durango, Old Mexico, 
but they have never been traced to any vein. So far as the geo¬ 
logical relations are concerned there seems no reason why tin 
might not be found in the Archaean granite rocks of Colorado. 
In England it occurs associated with granite, porphyry dykes, 
slates and quartz veins. The English “stream works” are placers 
derived from these in the same way as our gold placers are 
derived from rocks originally “in place.” 

CHARACTERS OF GOLD IN PLACERS. 

Nuggets of a large size are not common in gold veins, as 
they are in placers, yet they may exist in veins, for the largest 
American nugget, according to Newberry, was found in the vein 
of the Monumental mine at the Sierra Buttes, Downsville, Cali¬ 
fornia. It weighed ninety-five and a half pounds. Possibly in 
times long before man or mining, they may have been more com¬ 
mon in the veins than now. And again, as our mining opera¬ 
tions are but slight, they may be found hereafter. 


XXVll 


GEOLOGY OF COLORADO ORE DEPOSITS. 

Gold in placers is purer and of higher grade than that in 
veins, owing probably to its having been leached of its alloys by 
water and chemical action. 

Silver we do not find usually in placers, it having been 
destroyed by water, but such insoluble substances as magnetite, 
titanic iron, garnets, rubies and even diamonds are found closely 
associated with gold in placers. 

Gold is not wholly insoluble, but may be attacked by persalts 
of iron and salts of vegetation, so it may go through some chem¬ 
ical changes in placers, and some nuggets may be formed by 
concentration of gold in the placer itself. Nuggets of large size 
are, as a rule, found nearest the quartz veins which have sup¬ 
plied them, and the gold becomes finer as we recede from the 
source or from the mountain region. Pebbles of quartz contain¬ 
ing gold are common in placers, showing the origin of the gold 
from a quartz vein, sometimes at least. Nuggets show on their 
surface the battering they have received in the stream. 

CHARACTERISTICS OF PLACERS. 

“ Places where water currents were broken by a more moder¬ 
ate descent, sudden change of direction, or discharge of a side 
stream, are liable to receive gold deposits. Slight depressions, 
holes, open fissures or cracks in ‘ bed-rock ’ over which the cur¬ 
rent passed are often rich. The deepest layers near the bottom 
of the placer deposit or on the ‘ bed-rock ’ are generally richest. 
Periods of deposit may have followed one another and several 
rich layers lie one above another. Ancient as well as modern 
river channels may contain gold.” The prevalence of a certain 
peculiar or characteristic pebble in a placer may enable one some¬ 
times to trace the gold deposit back to the original locality 
whence the placer was principally derived and so lead to the orig¬ 
inal vein. 

In some localities, especially where the “ bed-rock ” happens 
to be jointed sandstone or limestone, the gold may find its "way 
for some little depth beneath the strata, and it becomes necessary 
to remove carefully a few feet of the bed-rock until a true “ floor,” 
such as an impervious layer of clay or other rock is found, below 
which experience proves the gold does not pass. The richest 


XXVlll 


GEOLOGY OF COLORADO ORE DEPOSITS. 


deposits of gold will often be found on that floor. The placer 
beds in Colorado consist of banks of pebbles of all. sizes, mingled 
with some sand and gravel, showing little or rude signs of strati¬ 
fication. These beds are from io to 50, sometimes 100, feet 
thick and form a series of rolling or undulating banks along the 
sides of our canons, valleys or watercourses. Gold is found from 
the grass roots to the bottom of the deposit, but principally near 
the bottom, and especially on bed-rock. Associated with the 
gold, usually near the bottom of the bank, we often find a rusty 
sand containing pebbles of magnetite iron known as “black 
sand.” This iron may have been derived from the original 
pyrites or “ blossom ” of the gold vein. 

SURFACE DEPOSITS.-BOG IRON. 

Of other surface deposits the commonest are those of bog 
iron, with which manganese oxide is sometimes associated. 
These beds, which are more or less impure, consist of hydrated 
peroxide of iron containing, when pure, 14.42 per cent, water. 
Phosphoric acid is sometimes present in quantity sufficient to 
diminish its value as an iron ore. Too much silica and other 
impurities may have the same effect. Bog iron ore frequently 
encloses the partially fossilized remains of roots of trees and 
swamp vegetation. The ore is the result of the chemical action 
of water, assisted by the acids of vegetation, upon minerals con¬ 
taining iron in another state, as upon iron pyrites and copper 
pyrites, as seen in the brown “gossan,” “ blossom ” or “ float ” of 
the outcrop of veins. Iron-bearing minerals such as mica, horn¬ 
blende, and augite, common in granite and eruptive rocks, con¬ 
tribute to these ores. 

A deposit of bog iron ore is found near Crested Butte, in a 
swamp situated upon a terrace or drift at the base of a mountain. 
The original source of this iron is traceable to a vein of iron 
pyrites up the mountain slope. The drainage of the mountain 
has passed through this vein, leached out the iron from the pyri¬ 
tes, and redeposited it in an oxidized and hydrated state in the 
swamp. The acids of the marsh vegetation have assisted in this 
chemical change, and in the precipitation of the iron, which is 
found enclosing the roots of trees and grasses. The ore is 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xxix 


remarkably good and pure. Its amount of Phosphoric acid is 
too high for Bessemer steel, but not for common pig iron. The 
amount of silica is very slight, while its percentage of peroxide 
of iron is very high. 

Analysis by Prof. Chauvenet, of School of Mines: 

Water and organic matter . . 23.97 per cent. 

Silica.2.50 

Peroxide of iron (iron, 50.73) . 72.47 “ 

Alumina.0.28 “ 

Lime.0.22 “ 

Magnesia.0.12 “ 

Phosphoric acid.0.333 “ 

Phosphorus.0.145 “ 

• 

Total.99.893 

IRON ORE IN COLORADO. 

Iron ores, when they occur in metamorphic crystalline rocks 
such as granite, are in the state of ferric oxide (hematite), or 
magnetite. These ores are found in metamorphic rocks of 
Archaean and Paleozoic age. 

Red hematite may be crystalline, fibrous, botryoidal, or com¬ 
pact. Magnetic iron ores containing much titanic acid are value¬ 
less. The magnetite from Grape Creek, near Silver Cliff, is an 
example. Fine-looking magnetites from several localities in 
Gunnison County are of no value, from their high per cent, of 
titanic acid. The black auriferous sands of California derived 
from the breaking up of eruptive rocks are titanic. 

“ Magnetite ” was originally deposited by water solutions as a 
common hydrated iron or limonite, and by heat of metamorphism 
was crystallized like the surrounding rocks into magnetite, in this 
way losing its combined water. 

In the oldest rocks, and especially the crystalline rocks, such 
as granites and porphyries, iron is a constituent of many of their 
component minerals, such as hornblende, garnet, mica, augite, 
etc. It is also the staining element in our feldspars, giving them 
their pink or red tint. In such rocks the ore is generally 








V 



XXX 


GEOLOGY OF COLORADO ORE DEPOSITS. 



magnetite when in granite, and hematite when in schist. Much of 
the common limonite and iron oxide found in unaltered sediment¬ 



ary sandstones along our foothills and plains was indirectly 
derived from these sources, for the elements of these unaltered 
sandstones consist of the detritus of granite and other older crys¬ 
talline rocks when the sedimentary rocks were in the condition 
of gravel or soil. The magnetite and hematite in the granite 
minerals being exposed to water, were changed into hydrated 
ferric oxide. In this condition as a red coloring of the soil it was 
exposed to carbonic acid and the acids of vegetation, and was 
finally deposited as a common limonite, or as a carbonate of iron 
(“ kidney iron stone.”) 

From observations in some of our iron veins in Colorado it 
would appear that iron pyrites is the original form from which by 
a secondary process, principally through surface action, magnetite 
and probably red hematite were derived, for we find magnetite 
on the surface passing down with depth into a vein of unaltered 
iron pyrites. “ The process may be thus: iron pyrites under 
surface action has its iron and sulphur oxidized, and passes first 
into iron sulphate and thence into iron oxide. If heat or meta- 
morphic action should now take place it is crystallized or changed 
into magnetite or red hematite.” 

As limonite and some carbonate of iron is so characteristic of 
the unaltered sedimentaries of our plains and foothills, and mag¬ 
netite and hematite of our metamorphic rocks of the mountains, 
we might consider the latter as metamorphic iron. Magnetite is 
the best and leading ore of Colorado at present. From Prof. 
Chauvenet’s reports we learn that— 

“ Iron is used for Bessemer pig iron, for steel and for common 
pig iron for foundry purposes*. 

The percentage of silica in an analysis should not exceed io 
per cent. 

Phosphoric acid is found in nearly all iron ore. For Bes¬ 
semer steel it should not exceed two-tenths of one per cent. (0.2 
per cent.) For common pig iron not over four-tenths of one 
per cent. 

The magnetites of Chaffee County, Colorado, are remarkable 
for their low per cent, of phosphorus. 


GEOLOGY OF COLORADO ORE DEPOSITS. xxxi 

Titanic acid, common in our magnetites, should not exceed 2 
per cent. 

Sulphur is deleterious to iron and usually occurs through the 
presence of more or less iron pyrites. 

Manganese is a good adjunct, is not common in Colorado, 
except in the Gunnison region, but is associated with iron ore in 
the Leadville mines. 

Water and organic matter, common in limonites and bog 
iron, can be easily gotten rid of. Much iron in the United States 
contains 8 to 10 per cent, water. 

Lime and magnesia are good in moderation. 

Iron ore should contain upwards of 50 per cent, metallic iron 
to be a good merchantable ore.” 

Along the foothills, beds of concretionary iron ore occur 
commonly above or below our coal seams of the Laramie creta¬ 
ceous. It is generally a limonite running too low in iron and too 
high in silica and phosphorus to be of use. With few excep¬ 
tions, so far we have discovered no promising iron beds along 
our plains or foothills. 

At the Trinidad coal field several thick belts of concretionary 
iron ore occur above the coal seams under Fisher’s Peak. The 
ore appears to be partially oxidized carbonate. Its analysis 
shows silica 9.19 per cent., protoxide of iron 45.04, lime 4.02, 
phosphoric acid 1.055, carbonic acid 33.035. Probably this is a 
type of other deposits along the coal strata of the foothills. 
4 ‘ Such ore might be used for smelting common pig iron. Its 
phosphorus percentage is too high for steel, but it might be util¬ 
ized as a mixing ore.” 

The marine dark shales of the Fort Benton group of the Cre¬ 
taceous, that is Cretaceous No. 2, yield locally, near Morrison 
and elsewhere, some dark heavy concretions, whose percentage 
of iron, not over 22, is too low for use, and its silica being also 
high. It is to the mountain, region we must turn for our great 
iron deposits. At Villa Grove we find irregular deposits of 
brown hematite in the metamorphic, Paleozoic limestone yielding 
58 per cent, iron, with only 0.031 phosphorus and no titanic 
acid. This is a good Bessemer steel ore. 


XXXll 


GEOLOGY OF COLORADO ORE DEPOSITS. 


The Calumet mine in Chaffee County is one of the best in 
Colorado. It is a great vein 40 feet thick, traversing syenitic, 
crystalline rocks, and yields 63.28 per cent, iron, with only 0.016 
phosphoric acid. It is largely used at the Bessemer Steel 
Works. In Park County, on Silverheels Mountain, near Brecken- 
ridge, a vein which produces excellent magnetite on the surface 
appears, with depth, to pass down into original iron pyrites, giv¬ 
ing by analysis : 

60.40 peroxide iron. 

22.12 protoxide iron. 

5.9 silica. 

No phosphorus and no titanic acid. This vein appears to 
illustrate what we have said of pyrites being the original mineral 
from which magnetite is derived. 

GUNNISON COUNTY IRON MINES. 

Perhaps the heaviest deposits of magnetite yet found in Col¬ 
orado are in Gunnison County. Of these the “Iron King,” near 
White Pine, is the most striking, owing to its great outcrop and 
partial development. Since, however, a description of these 
deposits is found in the report of Prof. Chauvenet, (issued simul¬ 
taneously with the present paper,) no details of occurrence or 
composition need be given here. 

MINERAL STAINS. 

Ores are sometimes disseminated through sedimentary rocks 
in which they have been chemically deposited. 

Some of our Triassic red sandstones in South Park are 
locally stained green with carbonate of copper. The sandstones 
contain impressions of fossil leaves beautifully colored with this 
material, but no profitable deposits of copper have been found. 
Copper stains also impregnate the hornblendic gneiss near 
Golden, and on a line or belt at various points along the eastern 
flanks of the mountains. Occasionally flakes of native copper 
are found, but prospecting has developed no profitable deposits 
of copper ore. 

Such blue and green stains of carbonate of copper frequently 
lead to the discovery of a true vein, containing copper or iron 


f 



London Fa.ul* 


South Park 


A 4 l > 


*>U* 




LD AND LONDON FAULT 1 


SHEEP MT FO 


OK HORSESHOE GULCH 


-sOUTH SID> 


]Vy>. WA/t* Porphyry^ rZ. ZJrch&a/i Grte/sr, 6 C<3/n Zr/3/J Qi/arlz/fe;C. 5/Zur/an i 


'//rrrsto/ra ; y. Af'c/J/e Carbo/fi/erirvs&rtts, e/.JtonerSjrd#/)///rat/* 2. isne sZ&r>e 


V^Mu's ~1L 















































4T> ‘I*- ma 

5<r> • wv\\t. ^ ^ •* • s'- 'v: •i.’ti -• •) . V-’, .\*>. 




































GEOLOGY OF COL OF A DO OFF DEPOSITS. 


XXX lit 


pyrites at depth, from which the carbonate stains have been 
derived by surface action. Such stains are common in every 
mining district, associated with the surface indications or “ float” 
of important veins or ore deposits. 

ORE BEDS IN COLORADO. 

“ Ore beds are metalliferous deposits interstratified between 
sedimentary rocks of all geological ages.” “They lie parallel to 
the planes of stratification and follow all the contortions of the 
enclosing strata, hence they are thrown into folds, troughs, 
arches, saddles, or basins. The upper portions of the arches may 
often have been removed by erosion, or the strata may be faulted.” 
The ore deposits or beds at Aspen occupy a faulted synclinal 
fold or basin. The enclosing rock is limestone, in part dolomitic. 
At Leadville the deposits occupy part of a series of faulted anti¬ 
clinal arches and synclinal troughs, of which the Mosquito range 
is the main axis. The beds lie between dolomitic limestone and 
sheets of porphyry. The ore beds partake of all the folding, 
faulting and other contortions which the enclosing rocks have 
suffered in the upheaval of the mountains. 

The thickness of such deposits varies much and may gradu¬ 
ally thin out and disappear, but may also continue long enough 
for all mining purposes. 

Often there are no sharp limits between an ore bed and the 
enclosing rocks, or between the ore bed and the walls, if walls 
exist at all. The ore appears to impregnate the surrounding rock 
by a chemical interchange between the elements of the rock and 
the ore. Such a “ metasomatic” interchange, “ substitution,” or 
“ replacement” appears to have taken place in the argentiferous 
lead deposits of Leadville and Aspen between the ore and the 
limestones. 

According to Phillips, “ a true ore bed never produces a 
‘combed’or ‘ribbon’ structure made up of symmetrical layers 
such as is common in so-called ‘true fissure veins,’ and is 
usually without the crystalline texture observable in veinstones.” 

FAULTS. 

Ore beds are subject to faulting, as at Leadville and Aspen. 
The common rule is that the footwall of the fault fissure usually 


XXXIV 


GEOLOGY OF COLORADO ORE DEPOSITS. 


rises or remains constant whilst the hanging wall falls down. 
When the opposite to this occurs.it is called a “ reversed fault.” 

The ends of the strata containing the bedded ore deposits on 
the footwall side of the fault fissure are commonly found bent 
downwards towards the fault as if dragged down by the fallen 
hanging wall side. In this case the ore deposit may be looked 
for below. The reverse, however, sometimes occurs. 

A faulted region is one in which great folding or crumpling, 
due to horizontal, lateral or tangential pressure, has taken place, 
and where the folding has reached its utmost tension, the fold has 
broken, and a slip or fault is the result. 

Thus in the South Park region adjacent to Leadville we find 
the horizontal strata of the Park as it approaches the Mosquito 
range becoming gently folded, the folds increasing in closeness 
and steepness the nearer they come to the range, until as >we pass 
up Four-Mile canon, which gives a complete cross-section of the 
Mosquito range, we find the axis of the range to be formed by a 
magnificent and very steep arch, breaking down abruptly into the 
great London fault, which runs along and splits the axis of the 
range for twenty miles. Still further on in the direction of Lead¬ 
ville we pass over a series of parallel faults, each one representing 
a steep fold that once preceded the faulting. Faults have their 
points of greatest depth, and die out at either end, often in folds, 
this being well illustrated at Leadville. Great faults are accom¬ 
panied by minor parallel faults, and also by cross faults intersect¬ 
ing them diagonally. 

INDICATIONS OF FAULTING. 

The surface indications of a fault are sometimes a “sag - ” or 
sudden depression in the outline of a hill. Thus from the Mos¬ 
quito range we descend to Leadville by a series of steps or 
benches, each marked by a sag on the hill slope, the depression 
often constituting a little ravine filled by a water course and 
abundant vegetation. 

In a mine such movements are indicated by slickensided or 
polished surfaces of the walls, or by a general broken up charac¬ 
ter of one wall or the other, due to the grinding of the walls in 
process of slipping. This grinding, as in the case of the great 


GEOLOGY OF COLORADO ORE DEPOSITS . 


XXXV 


Comstock mine, sometimes reduces quartz to a powder, like 
“commercial salt.” The polished slickened sides often show 
groovings or striae, indicating the direction of the slipping 
motion; such motion was probably slow or by short jerks, and 
may sometimes have been accompanied by earthquakes. 

In a good section of a cliff, such as is sometimes afforded by 
a deep canon, by observing some well defined or peculiar stratum 
high up the face of the cliff, we may notice its position out of 
place further down and easily compute the amount of slip, but 
when slips or faults amout to thousands of feet of displacement 
it is only by an accurate knowledge of the original geological 
position of the displaced members that we can form an estimate 
of the amount of slip. For example, if the strata of a period 
such as the Silurian, whose geological position is near the 
base of the series, should abut against rocks of the Tertiary, 
whose position is near the top, we should conclude that a slip of 
many thousands of feet had occurred. We should know that the 
Silurian had risen many thousands of feet and that the Tertiary had 
fallen correspondingly, so as to bring these two widely distant 
periods on the same level. By observing somewhere else the 
thickness of the strata composing the periods intervening between 
the Silurian and the Tertiary we should form an estimate of the 
amount o^ slip. 

On Castle Creek, near Aspen, the Mesozoic red strata, whose 
thickness numbers several thousands of feet, lies in the bed of 
the creek at the bottom of a lofty cliff of granite forming Aspen 
Mountain, upon the top of which rests the Silurian and Paleozoic. 
A great fault has therefore occurred by which the Paleozoic 
series has been lifted up and the Mesozoic series fallen down. 
The great fault fold of the Elk Mountains on Rock Creek in the 

o 

same region, is another striking example of how intense folding 
passes gradually into profound faulting. The downward direction 
of the fault fissure is commonly slanting rather than vertical, it is. 
not always regular, either; having sometimes a steeper dip in one 
place than in another, while its horizontal direction across the 
country is not always straight, but zigzag or curved. In a faulted 
district like Leadville there is generally a prevailing direction of 
up-throw or down-throw, thus at Leadville the up-throw is to the 


XX XVI 


GEOLOGY OF COLORADO ORE DEPOSITS. 


East which is also generally the footwall of the fault fissure, 
whilst the hanging wall or western face of the crack has fallen. 
The greatest amount of slip in that district amounts to some 
thousands of feet, and that is near the center of the district. 
The slip diminishing North and South and dying out in folds 
represented by round hills. 

The exact line of the fault fissure is generally obscured on 
the surface by crushed rock, debris and vegetation. In the mines, 
at depth we may find both walls wedged tightly together, and 
one side or both much crushed. Sometimes a little mineral may 
be found on the “cheeks” of the fault, dragged down from ore- 
bearing strata from above, or possibly leached from them and 
re-deposited. The fissure of the iron fault in the McKeon shaft is 
3 feet wide in places, filled with broken rock and a dark clay. 
The cheeks are altered by surface waters impregnated with 
iron oxide, and what ore there is is water-worn. The fault 
plane has been exposed for several hundreds of feet by the 
McKeon shaft. The outcrop of the fault on the surface is irregular, 
its direction downwards, is steep, but accompanied at intervals 
by benches with rich deposits of ore on them. The main fault 
is accompanied by a series of smaller faults adjacent to it. From 
the movement of timbers, by 5° or over, in some of the mines 
adjacent to the fault plane it is suspected that the fault movement 
is still continuing. The same suspicious facts have been observed 
in the Centennial mine at Georgetown. Nor is this improbable, 
since we have evidence of comparatively recent elevation of the 
mountains in various parts of the Rocky Mountain system. 

UNSTRATIFIED DEPOSITS, FISSURE VEINS, ETC. 

Mineral veins are changeable in character, and their appear¬ 
ances of a perplexing and complicated nature. There is a gradual 
passage from one form to another, so that it is difficult to classify 
them. There is often no such sharp distinction between one form 
of ore deposit and another as legal disputes would sometimes 
demand, and a witness should hardly be called upon to assert on 
oath that such a vein is a “ true fissure,” or another a “ bedded 
vein,” or a third a “segregated vein.” “Nature abhors straight 
lines ” and sharp distinctions, and delights in blending one form 
imperceptibly with another. 


GEOLOGY OF COLORADO ORE DEPOSITS. xxxvii 

Phillips divides veins into two classes, “ regular and irregular 
veins. Regular unstratified deposits include true veins, segre¬ 
gated veins, and gash veins.” “ Irregular deposits include 
impregnations, fahlbands, contact and chamber deposits.” 

Veins are collections of mineral matter, often closely related 
to but differing more or less in character from the enclosing 
country rock, usually in fissures formed in those rocks after the 
rocks had more or less consolidated. 

All veins do not carry metals; some are merely barren 
quartz, feldspar, or calcspar, like the barren veins we so often see 
traversing granite or limestone rocks. 

Veins may divide, “split up,” or thin out, and are irregular 
in shape and structure, owing to the irregular width of the fissures 
and to other causes. 

DEFINITION OF MINING TERMS. 

The rock in which a vein is found is called the “ country 
rock,” e. g., limestone, granite, porphyry. 

The portions of country rock in direct contact with the vein 
are called respectively the “ hanging wall,” or roof, and the “ foot 
wall,” or floor. This is only in inclined or flat veins, as a vertical 
fissure vein can have neither roof nor floor, but only two walls, 
East and West, or North and South, according to the compass. 
The inclination of a vein to the horizon is its “dip.” The hori¬ 
zontal direction of a vein at right angles to its dip is its “ strike.” 
The latter may commonly be observed along the surface outcrop, 
the former either in the workings of the mine or where the vein 
is exposed on the side of a canon. 

Both dip and strike of a vein often vary much, the former 
with depth, the latter with extension across the country. A vein 
or ore deposit will not unfrequently begin with a gentle dip, and 
increase rapidly in steepness with depth. The ore deposits on 
Aspen Mountain commonly begin with a dip of 25°, and at a 
depth of less than a thousand feet reach 6o° or more. 

As fissure veins commonly occupy fault fissures, their irregu¬ 
larities in dip and strike correspond to those we have already 
spoken about, under faults. 

The angle of dip is usually taken from its variation from a 


xxxviii GEOLOGY OF COLORADO ORE DEPOSITS. 


horizontal, not from a perpendicular line. Thus a dip of 75 
degrees means one that is very steep, while one of io degrees is 
a gentle inclination. 

A layer or sheet of clay called “ gouge ” or selvage often 
lines one or both walls of a vein between the country rock and 
the gangue or vein proper. It is derived from the elements of 
the adjacent country rock decomposed by water, and some¬ 
times by the friction of the walls of the fissure against one 
another, or against the vein matter, in the process of slipping and 
faulting, which is often shown by its being smoothed, “ slicken- 
sided,” polished or grooved. Gouge often contains some rich 
decomposed mineral in it, such as sulphurets of silver. It some¬ 
times occurs in the heart of a vein, especially if that vein has 
been re-opened anew by movements of the strata. The “ Chinese 
tallow” gouge of Leadville results from the decomposition of the 
feldspars in the adjacent white porphyry and is a hydrous silicate 
of alumina. 

In the granite veins in Clear Creek County the gouge is 
derived from the feldspars of the granite. Gouge is sometimes 
useful in defining the limit of the vein between walls, thus pre¬ 
venting unprofitable exploration into the “ country.” It is also a 
guide for following down a vein when mineral and gangue may 
be wanting or obscure. 

Both walls are not always clearly defined by slickensided sur¬ 
faces, by gouge or other mark, and so at times the vein is lost. 

False walls, caused by movements in the adjacent strata, by 
joints, etc., also mislead. 

It is not uncommon in Colorado for a fissure vein to have but 
one clearly defined wall, the other, if it exists, being obscured 
or changed by mineral solutions. Sometimes two cracks or fis¬ 
sures occur parallel to each other and the intervening country 
rock has been altered and mineralized into a vein. It is prob¬ 
ably in this way that many wide veins were formed. 

Mr. Emmons has found that fissures are formed by great 
movements of the earth’s crust or by local contraction of the 
rocks. That a fissure is not necessarily one with well defined 
walls, at considerable distances apart, filled after the formation of 
the fissure, but that the ordinary cracks or joints in granite quar- 


GEOLOGY OF COLORADO ORE DEPOSITS. 


XX XIX 


ries extending regularly to great lengths or depths illustrate the 
original fissures which have been changed by percolating waters 
carrying mineral solutions, into veins and deposits of ore. In all 
crystalline and sedimentary rocks these cracks or joints run 
parallel to each other at various distances apart, often plentiful 
and close together. In cases where percolating waters were 
charged with the proper metals and veinstone matter and the 
necessary chemical and physical conditions existed, the rocks 
lying between those cracks or joints were altered into ore. 

As one element was dissolved another took its place, so 
according to this authority it would seem that even a fissure vein 
may be only a sort of “ metasomatic replacement” of rock by 
mineral. Hence what is commonly accepted as a “ wall ” of a 
vein is not necessarily one, and cross-cutting, in order to deter¬ 
mine the lateral boundaries of the ore, is safer than to rely on 
supposed walls. A so called “ slip ” has often been followed by 
a miner as a supposed wall, until by accident he broke through 
and found good ore on the other side. If veins are formed accord¬ 
ing to Mr. Emmons’ theory, the occasional loss of one or both 
walls is easily accounted for. 

Cross veins of a more recent age sometimes cut or fault an 
older vein. The point of intersection is generally rich in min¬ 
eral. Cross veins must not be confounded with “leaders,” which 
are the filling of minor cracks extending off from the vein, and 
are .sometimes sufficiently profitable to work. While they some¬ 
times lead a prospector to the main vein, they may also lead a 
miner under ground astray from the true vein. 

The splitting of a vein by a “ horse” or large fragment of the 
country lying in the vein may be mistaken for a true cross vein, 
or the original fracture of the fissure may have been in the form 
of a star or like the spokes of a wheel radiating to the hub. In 
such cases there are no true cross veins. But when, as in the 
San Juan district, we have two well defined sets of veins, one 
striking North East by South West, and the other North West 
by South East, they cut each other diagonally, the cut vein being 
the older. These opposite sets of veins have been formed at 
different times. Many contain a characteristically different class 
or variety of minerals. Thus in Cornwall, England, one set car¬ 
ries tin and the other lead. 


xl 


GEOLOGY OF COL OF ADO OFF DEPOSITS . 


SIGNS OF A TRUE FISSURE VEIN. 

True fissure veins show signs of motion or slipping on the 
sides of the fissure, such as slickensides, gouge, crushed walls, 
“ horses,” or “ breccia,” the latter being small portions of the 
country rock fallen into the vein and cemented by vein matter. 
In the Comstock, the quartz is ground to powder. The vein 
itself, though occupying a healed fault fissure, may be itself faulted 
by later movements in the mountain after the vein was formed. 
Some of the fissure veins on Engineer Mountain, San Juan, are 
so dislocated. 

The vein-filled fissures being a line of weakness, may be 
re-opened by mountain movements, and other or different com¬ 
binations of ore introduced into the heart of the vein. Such a 
re-opening would be marked by a succession of “combs” or 
banded ribbon-like deposits of ore, and by gouge matter. 

OUTCROP OF VEINS. 

The outcrop of a vein is that which appears at the surface 
and usually attracts prospectors to the spot. Sometimes it may 
be, as in the San Juan district, a bold vein of hard white or rusty 
quartz, standing up in relief, by its superior hardness, above 
the surrounding country, like a low wall. Or again, in the same 
district, from being composed of softer or more soluble sub¬ 
stances than the prevailing eruptive lava sheets, instead of a wall 
it causes a depression or trough on the side of a hill forming the 
pathway for a rivulet and marked by luxuriant vegetation. Com¬ 
monly the outcrop consists of a decomposed mass of rock 
stained with oxide of iron and streaked here and there with 
green or blue carbonate of copper, and is called “ float” or “ blos¬ 
som” by the miners. This “float” is the chemically changed or 
oxidized portion of the true and unchanged vein lying deeper 
below the soil. 

In this “blossom rock” free gold is not unfrequently found, 
but unaltered sulphides, such as galena or iron pyrites, are rarely 
met with on the outcrop. In the San Juan district, on Mineral 
Point, we have, however, found galena at the grass roots, and 
broken off large chunks of it from a quartz vein outcropping on 
the surface, 


GEOLOGY OF COL OF ABO ORE DEPOSITS . 


xli 


In gold-bearing veins such an oxidized condition is desirable 
if it continue down to any depth, for so far as it continues the 
gold is free, and the ore is a free milling one, easily treated, and 
often exceedingly rich in gold, as in the celebrated Bowen mine 
of Del Norte; but as soon as the hard white quartz and the 
unoxidized pyrites of the true vein is reached, the ore is no longer 
free milling, but must be smelted. The gold may still be found 
tree, perhaps, in the hard quartz, but if the pyrites should not 
prove rich in gold, the palmy days of the mine may be considered 
as past. Many such rich deposits on the surface, abounding with 
specimens of free gold, have proved great disappointments with 
depth. 

WIDTH OF VEINS. 

Veins may vary in width or thickness from a half inch to a 
hundred feet. They also pinch or widen at intervals in their 
downward course. The widest “ mother ” veins are not always 
the most productive, though they are very persistent in length, 
and we may suppose in depth also. In the San Juan district 
the “mammoth” \*eins of quartz, often a hundred feet wide, 
are not the favorites for development, the ore being found too 
much scattered in them, and the development less easy than in 
those io, 20, or 30 feet wide, where the metal is more concen¬ 
trated. These mammoth veins in the San Juan are easily trace¬ 
able for miles over the surface of the country and down the sides 
of the deep canons. Their limiting depth has never been reached, 
and probably never will be by mining. 

DEFINITION OF TRUE FISSURE VEINS. 

True fissure veins are popularly defined as filling fissures of 
indefinite length and depth, commonly occurring in parallel sys¬ 
tems, traversing the surrounding rocks independently of their 
structure or stratification, and commonly, though not necessarily, 
at an angle different from that of the stratification — in other 

o 

words, cutting across the planes of stratification. These veins 
originated in fissures, not necessarily wide open ones, but on the 
contrary, rather narrow cracks, descending, however, to great 
depth such as those produced by faulting, or the general cleavage 
lines of the mountain. The latter may be frequently observed in 


xlii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


every canon, and also in the sedimentary rocks of the foothills 
and even along the flat surfaces of the plains. They are very 
conspicuous in the plains around Trinidad, and are there not 
unfrequently occupied by a series of narrow parallel dykes of 
basalt instead of by mineral veins. Cleavage lines or joints are 
familiar to every stone-quarry man. 

These cracks are caused by extensive movements of the 
earth’s crust in the process of mountain uplift, and also on a 
smaller scale by contraction of the rocks in cooling from a heated 
or molten condition, or even in consolidating from a soft or 
muddy condition. 

The two walls enclosing a vein do not generally coincide, as 
might be expected, since the vein occupies a line of fault. A 
true fissure vein may in some part of its course coincide with 
the dip of the surrounding strata. As the plane of stratification 
or line of division between one stratum and another is a natural 
line of weakness, a crack once started would be liable to follow 
it for some distance. And when uplift occurs such places are 
liable to slip one upon the other and a true parting fissure ensue, 
conformable to the prevailing dip. Such a vein might appear at 
first to belong to the class of so-called “bedded veins,” but if with 
depth it should be discovered to be cutting across the strata it 
would be pronounced a true fissure vein. The appearance of 
slickensides or other signs of motion on the walls of the apparently 
“bedded portion ” would then prove it to belong to the “true 
fissure” class and that actual Assuring had taken place prior to 
the vein-filling. 

CAUSE OF POCKETS IN FISSURE VEINS. 

As a fault fissure in its downward course usually pursues a 
zigzag rather than a straight course with smooth surfaces on 
either side of the crack, the inequalities of one face of the crack 
are brought into opposition to the inequalities on the other face 
as one or the other side of the fault slips up or down, and thus 
are produced pinches and wide cavities, which give rise to the 
“pinches” and “bonanza,” “pockets” so common in fissure veins. 

A so-called true fissure vein may sometimes have advantages 
over some other forms of vein occurrence, from its persistency 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xliii 


and comparative regularity to great depths. It must not, how¬ 
ever, be expected that it will continue equally rich or equally 
poor throughout its course. There, may be comparatively barren 
spots and rich spots, pinches and widenings, local combinations 
of richer or poorer varieties of mineral. But the vein as a rule 
is not likely to entirely give out. 

RICHNESS WITH DEPTH. 

There is no scientific reason why a vein should “ grow in 
richness and size with depth.” This is a popular fallacy, origin¬ 
ating from the now less accepted theory that veins were formed 
by the precipitation of precious metals by heated rising waters 
or vapors, and hence that the greater concentration would take 
place at greater depths. The “lateral secretion” theory now 
commonly accepted ascribes the deposition of ore to solvent 
waters reaching the vein from ground quite near to it and coming 
naturally from above and the sides quite as often as it is ejected 
upward by pressure from below. 

In Idaho Territory, says Mr. A. Williams, “ the rule is rather 
that veins grow less rich and strong with depth, though strong 
veins may continue metalliferous to a greater depth than mining 
can ever be reached.” 

“ The thickness of the earth’s crust which we are able to 
explore is very limited. Increase of heat, as in the deep Comstock 
mine, and other natural difficulties, limit us to a few thousand 
feet—3,000 at most. These deep mines have not, as a rule, 
proved richer with depth, but to the contrary. Some veins have 
been worked through alternate zones of richness and barrenness. 
The Comstock, which has been opened for four miles in length 
and to a depth of 3,000 feet, shows the ore bodies to be scattered 
irregularly and the barrenest ground is as the bottom. On the 
other hand some of the most celebrated mines derived their 
wealth from rich ores encountered near the surface and have 
proved most disappointing with depth.” 

Atmospheric action for a long period has often reduced the 
ore to its richest compound, and when the hard material is reached, 
leanness sets in. This, as we have observed, is commonly the 
case with gold veins. The richness of the Leadville mines is 


xliv 


GEOLOGY OF COL OF A DO OFF DEPOSITS. 


derived from their decomposed compounds. Again, as the sur¬ 
face crust can be so little explored by mining, it is to be remem¬ 
bered that the erosion by glaciers and waters has already removed 
thousands of feet of the vein, so that we are able to examine only 
a small fraction of it while an unknown quantity lies in the depths 
below. If these veins, then, continue to the supposed great depths 
below, we are very far from their starting point, and erosion having 
removed their upper portions, we cannot find their surface finishing 
point; in other words, it is not a fresh “ready made” vein we 
find, but portions of an old vein already extensively mined by the 
processes of nature. 

So far as our experience goes in Colorado, after a moderate 
depth is reached below surface action, or below the “ water level,” 
a fissure vein may grow richer or poorer, wider or narrower with 
depth, without any law except local experience in a district. 

VEINS IN GROUPS. 

Fissure veins occur in clusters and nearly parallel groups, 
forming a mining district, and again in that district certain pecu¬ 
liar veins may be. grouped together, forming a “ belt.” Thus 
Boulder district occupies a certain isolated area, outside of which 
few mineral deposits occur for a long distance. We have also in 
that district several distinct belts carrying different characteristic 
ores, such as the telluride belt, marked by rare telluride deposits, 
the pyritiferous gold-bearing belt, and the argentiferous galena 
belt. The Central City region is characterized by auriferous 
pyrites belts, Georgetown district, not far distant, by argentiferous 
belts, and Idaho Springs, lying between the two, by both gold 
and silver belts. 

t 

RELATION OF VEINS TO ERUPTIVE FORCES. 

The ultimate cause of the richness in veins of a district or 
locality is, that local dynamic and eruptive forces were more ener¬ 
getic there than elswhere, causing great disturbance of the rocks 
accompanied by fissures, and eruptions of porphyry. 

Thus at Leadville the Mosquito range is violently folded and 
fractured/eruptive rocks have issued abundantly, and associated 
with such phenomena we find great lead and silver deposits. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xlv 


Further South the great San Juan district is split up in an 
extraordinary manner with great fissure veins. The region is an 
eruptive one, consisting of prodigious flows of porphyry or erup¬ 
tive rocks traversed, not unfrequently, by newer eruptive dykes. 

In the Gunnison district the strata have been overturned, dis¬ 
turbed, folded and faulted in an extraordinary manner by the 
intrusion of great masses of eruptive rock forming the peaks of 
the Elk Mountains. The strata every where are riddled by dykes 
or intrusive sheets, and the evidence of heat is apparent in the 
general metamorphism of the entire region. Mineral veins abound. 
The same phenomena are repeated more or less- in the neighbor¬ 
ing region around Aspen, at Pitkin, and at Tincup. 

At Boulder, Central and Georgetown there is a concentration 
of eruptive dykes locally in each district and few dykes or erup¬ 
tive rocks outside of those districts. On the other hand we have 
no ore deposits in the undisturbed rocks of the plains or the flat 
basins of our parks, and notably our mining districts are for the 
most part well in to the core of the mountains, where, in the 
nature of things, folding, crumpling, faulting, eruptions and meta- 
morphic heat were more energetic than along the flanks and foot¬ 
hills of the range which have usually proved unproductive. 

The older eruptive rocks such as the quartz, porphyries and 
diorites of the Leadville, South Park and Gunnison districts, are 
more favorable to the production of ore deposits as a rule, than 
the more modernly erupted lavas, such as basalt or dolerite which 
we commonly find occuring in dykes and surface overflows trav¬ 
ersing or capping our Cretaceous and Tertiary coal fields along 
the foothills as at the Table Mountains at Golden and Trinidad. 

Some of the lighter colored and somewhat recent lavas like 
the tufaceous rhyolite, which caps so many of the Tertiary mesas 
on the divide between Denver and Colorado Springs have also 
hitherto proved barren. The older eruptive rocks, as we have 
stated, are nearly all of an intrusive character, never having 
reached the surface, while the newer ones bear evidence of having 
flowed over the country like modern lava streams, as is shown by 
spongy scoria on their surface, and may be called “extrusive.” 

In Colorado the ore body is not usually found in the heart 


xlvi 


GEOLOGY OF COLORADO ORE DEPOSITS. 


of an eruptive sheet or dyke of porphyry, but at the line of its 
contact with some other rock, such as limestone, granite or gneiss. 

CONTACT DEPOSITS. 

The “contact” ore deposits of Leadville occur at the contact 
of quartz, porphyry and dolomitic blue limestone. 

Some of the veins at Boulder, Central and Georgetown are at 
the contact of porphyry and granite or gneiss. 

Exceptions occur, however, where mineral is found either in 
the heart of a dyke, or the whole dyke may be so impregnated as 
to constitute in a sense a vein. These exceptions are generally 
confined to pyritiferous gold deposits. 

GOLD-BEARING DYKES. 

Suppose a dyke or mass of eruptive rock to be thoroughly 
impregnated with gold-bearing pyrites. Near the surface and 
often for a considerable depth the rock is decomposed and the 
pyrites oxidized into rusty iron ore, liberating the gold which is 
entangled in the “gossan” in wires, flakes or even small nuggets. 
As long as this decomposed or oxidized state continues, the ore 
is free milling, but with depth the dyke is found in its primitive 
hardness, studded with iron pyrites which may or may not prove 
rich enough for the more expensive treatment of smelting. Such 
gold-bearing dykes are found at Breckenridge, South Park, also 
in Idaho Territory and in old Mexico, and many other gold-bear¬ 
ing regions. 

The Printer Boy gold mine at Leadville is a vertical deposit 
in a jointing or fracture plane in a dyke of quartz-porphyry, rusty 
and much decomposed near the surface, where it yielded free gold; 
with depth this passes into copper and iron pyrites. The vein is 
from an inch to four feet in width, stringers carrying ore extend 
into the porphyry, which is highly charged with pyrites which 
doubtless supplied the vein with mineral through the agency of 
surface waters. In Arizona, near Prescott, at the Lion mine we 
find a green dyke of eruptive diorite penetrating granite. This 
dyke is traversed by numerous small veins of white quartz which 
near the decomposed and rusty surface are rich in free gold. At 
a slight depth the quartz veins become charged with unoxidized 


GEOLOGY OF COL OF A BO ORE DEPOSITS. xlvii 

iron pyrites sufficiently rich in gold to merit treatment by smelt¬ 
ing. The surface ore is treated by a simple “arrastre,” and is, of 
course, free milling. The gold seems to be mostly confined to 
the quartz veins. 

FISSURE VEINS IN IGNEOUS AND GRANITE ROCKS. 

The San Juan district is an exceptional case where immense 
numbers of fissure veins penetrate igneous eruptive sheets. The 
fissure veins consist of hard gray jaspery quartz, traversing lava 
sheets whose united thickness is from 2,000 to 3,000 feet. The 
veins produce lead, bismuthinite, gray copper and other silver¬ 
bearing ores. 

In Colorado true fissure veins are most characteristic of the 
Archaean granitic series. In fact, all the veins in that series are 
fissure veins. Locally they occur as in the San Juan, cutting 
through eruptive rocks. Outside of these formations few true 
fissure veins occur. 

An exception may be made of the Gunnison and Elk Moun¬ 
tain region where the fissures traverse all the formation from 
Archaean granite to the top of the Cretaceous coal beds. Nearly 
all other mineral occurrences, such as those in the limestone 
regions, come under the class of bedded-veins or blanket-veins, 
pipe-veins or “pockets” and show none of the characteristics of 
slipping motion or fissure action. Under this latter class the 
Leadville and Aspen deposits may be grouped. 

Ore deposits commonly occur at the junction or contact of 
two dissimilar rocks, as between quartzite and limestone or lime¬ 
stone and dolomite. 

Lodes occur also between the stratification planes of the same 
class of rock, sandwiched in between two layers of limestone, and 
sometimes impregnating the layers on either side for some distance 
from the dividing line between the two stratas, which is commonly 
the line of principal concentration of ore, and often descend 
from this concentration line, through the medium of cross joints, 
to form large pockets in the mass of the limestone. The Aspen and 
Leadville deposits are of this character. Also when ore bodies 
occupy a true fissure i. e. one cutting across the stratification 
planes, they may locally, for a short distance, impregnate the 


xlviii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


adjacent walls or country rock more or less. Our fissure veins in 
granite and gneiss often impregnate the walls to a small extent. 

Mineral deposits favor as a rule the older rocks, such as the 
Archaean and Paleozoic series, probably because heat and meta- 
morphic action are commoner in these older rocks, which have 
felt all the throes of the earth from past to present times, than in 
the more recent ones, and such circumstances, as we have stated, 
are peculiarly favorable to vein formation and mineral deposition. 

The bulk of our precious minerals in Colorado come from the 
older Archman and Paleozoic series of rocks, the exception being 
the Gunnison region around Crested Butte, Irwin and Ruby, 
where ore comes from fissure veins in the Mesozoic Creta¬ 
ceous rocks. The exception is accounted for by the local meta¬ 
morphism, heat and eruptive phenomena of that region. 

The veins in the San Juan have also been ascribed by some to 
the Tertiary period, owing to their occurrence in certain supposed 
Tertiary lavas covering that district. 

Besides heat, metamorphism, dynamical disturbances and 
eruptive agencies, other minor circumstances may favor ore depo¬ 
sition. Certain rocks, such as limestones, may offer by their ten¬ 
dency to solubility and chemical reactions, more favorable condi¬ 
tions than others for mineral solutions to deposit by “ metaso- 
matic” interchange between mineral and limestone, until the 
limestone is gradually replaced by ore, much in the same way as 
the elements of a water-logged trunk of a tree are replaced by 
silica in the process of fossilization. 

DESCRIPTION OF ERUPTIVE ROCKS. 

As eruptive igneous rocks commonly called “ porphyries” 
play so important a part in connection with ore deposits, it will 
be well to give such a general description of them as will enable 
a miner to recognize and distinguish them in the field. 

They differ from ordinary sedimentary rocks, such as sand¬ 
stones and limestones, in their origin, mode of occurrence, struct¬ 
ure and appearance. 

Their origin is that of fire, not of water. They have been 
thoroughly fused and melted in the bowels of the earth. They 
occur erupted through fissures, piercing both the foundation 



SUMMIT OF MT LINCOLN 

AND NORTH WALL OF CAMERON AMPHITHEATRE (UPPER END) 

JbJrtfimv. i.Cnmbr, 3 n Quirtzrte o.Si/or ^Lo^rCarboaiferaus Lynestnne; MoMlheMr^yryy tyMb/hpAys/ 































































* 




r 











VmYnC *3 . \ ■s.Vufcv*^ «* vV Vi 


\ 





























GEOLOGY OF COL OF A DO OFF DEPOSITS. 


xlix 


granite and overlying quartzites, limestones or sandstones, prying 
open the leaves of the sedimentary strata, and forcing their way 
in between the strata in horizontal sheets, arching up the strata 
and faulting them and filling the space they have opened with a 
thick, dome-like, lenticular mass of porphyry sometimes 1,000 
feet thick, called by geologists a Laccolite, which ultimately thins 
out either way in a sheet terminating in a wedge between the 
strata, showing their eruptive intrusive character and their igneous 
origin. The internal structure of these rocks is thoroughly crys¬ 
talline, as distinct from those of sedimentary aqueous origin. 
The latter show either to the naked eye or under the microscope 
that they are made up of more or less water-worn fragments of 
other rocks. 

The eruptive rocks are of course unfossilliferous by reason ot 
their igneous origin. They are, as a rule, harder than most of 
the sedimentary rocks and will not generally split up into lamina 
like shale, slates, or some sandstones. There is no “ way of the 
grain” to them as there is in those of aqueous origin. They 
break like cast iron or other crystalline substances, and both on 
the surface and fracture show their distinctly crystalline char¬ 
acter. 

CHARACTERISTICS OF PORPHYRIES. 

The component minerals of these intrusive porphyries are 
principally quartz and feldspar, together with mica or hornblende. 

In color, these rocks are commonly some shade of gray or 
green, maroon, or even white, but their most striking characteris¬ 
tic is a general spotted appearance. This arises from large per¬ 
fect crystals of quartz or feldspar being set in a finer-grained 
crystalline paste or background, and standing out prominently 
from it. This base or background may be comparatively coarsely 
crystalline, finely crystalline, or so finely crystalline that the crys¬ 
tals can only be discovered by the microscope, and the larger 
crystals of feldspar seem set in the paste, like plums in a pudding. 
In the depths of a mine the porphyry is commonly much decom¬ 
posed, and even passes into “ gouge ” or clay matter. Its spotty 
character, from the presence of individual feldspar crystals, even 
then may identify it at times, or the aluminous character of the: 
decomposed rock may be sufficient. 




l GEOLOGY OF COLORADO ORE DEPOSITS. 

When feldspar is the main constituent it is called a felsite-por- 
phyry. When a certain amount of quartz is observable, a quartz- 
porphyry. 

Diorite also belongs to this intrusive eruptive class, differing 
mainly in the fact that its feldspar is plagioclase instead of ortho- 
clase. Hornblende is generally a prominent constituent, and gives 
it commonly a more or less dark green or greenish hue. In 
appearance it is not unlike common granite or syenite, but its 
eruptive occurrence distinguishes it at once from them. 

The main peaks and dome-like masses of the Elk Mountains 
are principally diorite, which in old reports used to be called 
“ eruptive granite.” It is generally finer grained than the com¬ 
mon quartz porphyries. From the dark olive green that it 
assumes when weathered it has been called “ greenstone.” 

MODE OF OCCURRENCE OF QUARTZ PORPHYRIES IN SOUTH PARK. 

Quartz porphyries are among the commonest varieties of 
intrusive eruptive rocks in the mining districts of Colorado. The 
eruptive rocks of the Leadville and South Park districts are prin¬ 
cipally quartz porphyries. In almost any canon in the Mosquito 
range between South Park and Leadville we get a section of a 
thousand feet or more of different rocks, and we see the canon 
cliffs to be composed at the base of granite and gneiss, upon 
which rests, at a slight angle, a great thickness of sedimentary 
beds. These are principally quartzites and limestones. Passing 
up through the granite, we may notice a dyke of quartz porphyry, 
which perhaps when it enters the sedimentaries, opens them up 
between their strata and intrudes itself in a mass, gradually run¬ 
ning out at either end. The dark green or gray color of the 
eruptive rock, together with its columnar structure, readily dis¬ 
tinguishes it from the sedimentary beds. Sometimes a great 
“ laccolitic ” mass is so formed between the strata, which are 
arched up and faulted. At other times the eruptive sheets look 
almost like interstratified rocks, so well do they conform to the 
strata. Its intrusive character, however, is very apparent by its 
tendency to cut across from one set of strata to another. The 
feeding dyke or vent of this porphyry may not be seen always 
attached to its laccolite, sheet or branches, but may be found per- 


GEOLOGY OF COLORADO ORE DEPOSITS. 


li 


haps miles away as a dyke coming up through granite from the 
top of which the sedimentary rocks, together with their included 
intrusive sheet, may have been entirely eroded off, and we have 
to look for the porphyry sheet elsewhere. Sometimes, again, we 
may find the great laccolite entirely denuded of the strata which 
once arched over it, and it may constitute, as in the case of the 
principal peaks in the Elk Mountains, a prominent peak or dome 
of eruptive rock. Crested Butte Mountain, Gothic Mountain, 
both of quartz porphyry, and White-rock and Snow-mass Moun¬ 
tains, of diorite, belong to this laccolitic type—reservoirs of con¬ 
gealed eruptive rock, revealed by the denudation of thousands of 
feet of sedimentary rock, that once arched over and lay above 
them. Whenever these porphyries or diorites are found in Colo¬ 
rado their presence implies that great denudation has occurred, 
uncovering these deep-seated subterranean reservoirs of molten 
rock, for, as we said, they are all intrusive sheets. None of them 
ever flowed out over the surface, but they came up from depths 
unknown, and not having eruptive energy enough to penetrate 
to the surface of the earth, they spread out and congealed beneath 
that surface, forcing their way into any local weakness among 
the adjacent strata. 

A dyke not unfrequently is the cause of the existence of a 
prominent mountain peak. Thus Mount Lincoln over 14,000 
feet above the sea, owes its prominent character to a dyke of 
quartz-porphvry which has come up through the granite and sent 
out intrusive sheets between the overlying quartzites and lime¬ 
stones binding the mountain mass together as by a tree with 
locking branches and so preserving it from the general erosion as 
a noble monument of fire and water. These eruptions appear to 
have occurred at different times, as we frequently meet older 
intrusive sheets cut by dykes and intrusive sheets of a newer and 
a different variety of porphyry. The eruptions in the South Park 
rep-ion seem to have occurred till near the close of the Mesozoic 
epoch, as these dykes and sheets are found in rocks of that epoch 
as well as penetrating the older Archman and Paleozoic series. 

The eruptive rocks we have mentioned are those most com¬ 
monly associated with our ore deposits, but there is another and 


lii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


apparently more recent class less commonly associated with ore 
deposits in this State; these are, 

YOUNGER EXTRUSIVE IGNEOUS ROCKS. 

Their leading characteristic is that they have reached the sur¬ 
face and flowed over it like modern lavas and may be called 
extrusive rather than intrusive. Typical of these we may cite 
the dark dolerites and basalts that cap our coal fields and Ter¬ 
tiary and Cretaceous strata along our foothills, frequently forming 
mesas or table lands by their protecting cap of lava, or appearing 
in steep ridges or sharp conical hills called “buttes” where their 
dykes only are exposed. The andesite forming Buffalo Peaks, 
South Park and the tufaceous pink rhyolite capping the Tertiary 
mesas on the divide between Sedalia and Colorado Springs, so 
much used for building stone, and the rhyolites of Nathrop and 
Chalk Mountain, Kokomo, belong to this extrusive or more 
modern type of igneous rock. 

The colors of these rocks are either very dark gray or black, 
or very light, and even white. In the one case they consist of 
dark, heavy minerals such as augite, magnetite and feldspar and 
are said to be basic. In the other, principally of light minerals, 
such as feldspar and quartz, with a sprinkling of mica or perhaps 
a little hornblende and are called acidic. 

The lighter rocks are popularly and erroneously called “trach¬ 
ytes.” They are mostly rhyolites, as trachyte is a very scarce 
rock in Colorado or in these mountains generally. 

At Nathrop station near Buena Vista and Chalk Mountain, 
Kokomo, and Black Mountain, South Park, are good examples 
of rhyolites. The great eruptive region of the San Juan appears 
to consist of quartz-porphyries, diorites, porphyrites, some ande¬ 
site, rhyolites and basalts so far as we at present know, and there 
appears to be an older and a n£wer series of igneous rocks in that 
region. 


TYPICAL PORPHYRIES AND ERUPTIVE ROCKS DESCRIBED. 

Though there are a great variety of eruptive igneous rocks 
distinguished from one another by many hard names and subtle 
definitions, the ordinary mining man need concern himself with 


GEOLOGY OF COL OF ABO OFF DEPOSIT'S. 


liii 


\ 

but a very few characteristic types commonly met with near the 
ore deposits of Colorado. We will mention some of these and 
roughly define them. 

They are quartz-porphyries, porphyrites and diorites. 

A quartz-porphyry is a porphyry that contains quartz crystals, 
large or small, in addition usually to very large crystals of ortho- 
clase feldspar, generally of a vitreous or glassy variety called 
“Sanidin,” together with small crystals of hornblende or mica. 
We will take that which forms the dyke on Mt. Lincoln, South 
Park, as a type. It is called Mt. Lincoln quartz-porphyry. 

In appearance it is a gray rock, spotted with large crystals of 
orthoclase sanidin feldspar, which sometimes shows an oblong face 
two inches long by nearly an inch wide, at other times a shape 
like the gable end of a house, according to which part of the 
crystal is exposed. Sometimes two crystals are seen locked 
together, forming what are called Carlsbad twins. When the 
rock is decomposed these crystals not unfrequently fall out, and 
lie as pebbles on the ground. With these may also be seen the 
ends of bluish or pinkish crystals like broken glass. These are 
portions of quartz crystals which, when extracted perfect from 
the rock, show a six-sided pyramid at either end. These large 
crystals are set in a crystalline paste of smaller crystals of the 
same kind, together with many little black cubes of shining mica 
or duller lustred and longer rectangular oblong crystals of horn¬ 
blende. 

This porphyry is eruptive and intrusive, occurring in dykes 
and intrusive sheets and laccolites. In the South Park region 
around Mt. Lincoln, and in the Gunnison region, around Crested 
Butte and Gothic, it is exceedingly common, as well as in many 
other localities in Colorado. In Leadville there are several 
varieties of much the same class of rock. 

LEADVILLE WHITE PORPHYRY. 

At Leadville there is a quartz-porphyry known as the Lead¬ 
ville white porphyry, or “ block porphyry,” by the miners, which 
needs description, as it is the one that commonly overlies the 
rich ore deposits. It is a white compact homogeneous looking 
rock not unlike a slialy white sandstone, limestone or quartzite. 


liv 


GEOLOGY OF COLORADO ORE DEPOSITS . 


It consists of feldspar, quartz and a little mica. Its porphyritic 
or spotted character is so indistinct that one would hesitate to 
call it at sight a porphyry, but the microscope reveals perfect 
double pyramids of quartz and perfect individual crystals of feld¬ 
spar set in a paste of the same minerals. It is often stained by 
concentric rings of iron oxide and marked with wonderful imita¬ 
tions of trees. The latter have earned for it the title of “ photo¬ 
graphic rock.” These markings are only the crystallization 
forms of oxide of iron or manganese, something after the manner 
of fern frost work on a window pane, and are called “ dendrite,” 
or tree rock. This porphyry is very shaley and breaks off into 
thin blocks, hence its name locally of “ block porphyry.” It is 
an eruptive, intrusive rock occurring in dykes, laccolites and 

intrusive sheets. It is common at Leadville, but not so much so 

• 

in other parts of Colorado. There are several other quartz por¬ 
phyries akin to these, such as the gray porphyry, the Sacra¬ 
mento porphyry and the pyritiferous porphyry. The latter is so 
named from its being everywhere highly charged with minute 
particles of iron pyrites. It is not improbable that a good deal of 
the gold of this region came indirectly from this porphyry. 

PORPHYRITE AND DIORITE. 

These two rocks are so nearly alike in their mineral compo¬ 
sition, mode of occurrence and appearance that we class them 
here as one. They differ from quartz-porphyries mainly in the 
fact that their feldspars belong to the plagioclase instead of the 
orthoclase variety. Hornblende also is a main constituent of 
these rocks and gives them their general dark gray or greenish 
tint. The appearance of diorite is not unlike that of granite or 
syenite, but its mode of occurrence as an eruptive, intrusive rock 
distinguishes it from the latter. Quartz is also present, so their 
general composition may be plagioclase'—feldspar, quartz and 
hornblende. Mica, too, may or may not be present. A porphy- 
rite appears to be little more than a porphyritic or spotted diorite. 
The plagioclase crystals are much smaller than the orthoclase 
feldspars of the porphyries and generally of a glassy white color. 
A hand specimen of porphyrite might readily be mistaken for 
granite but its spotted appearance can generally be detected, 


GEOLOGY OF COLORADO ORE DEPOSITS. Iv 

distinguishing it from the latter. Hornblende decomposing gives 
these rocks an olive green color, hence their name of “green 
stones.” 

Porphyrite may be observed forming intrusive sheets in the 
walls of several of the canons of South Park, such as Buckskin 
or Mosquito canon. 

Diorite constitutes some of the principal eruptive peaks of the 
Elk Mountain range, such as White-rock Mountain, and the 
rocks along Copper Creek near the Sylvanite mine. A great 
mass of quartz-mica-diorite much decomposed and spotted with 
iron pyrites overlies the ore-bearing limestone in Vallejo gulch, 
in Aspen Mountain, and is traceable for many miles in the direc¬ 
tion of the Elk Mountains, from whose eruptions it was doubtless 
derived. 

These are the principal eruptive intrusive rocks met with in 
our mining districts, and we pass over the “ extrusive” eruptive 
rocks, such as the dark basalts and dolerites, the pale rhyolites 
and dark andesites, because they are rarely found associated in 
any important way with our ore deposits. 

METAMORPHIC ROCKS. 

There is another class of rocks, however, which the miner 
meets with which are generally more or less crystalline in char¬ 
acter but are neither eruptive nor intrusive. Their crystalline 
character, however, shows they have been subjected to a certain 
amount of heat. These are called metamorphic or changed 
rocks, because they were originally common sandstones, shales 
or limestones, such as we find underlying the prairie, and have 
been partially changed or metamorphosed by heat into a crystal¬ 
line condition very unlike their original one. To this class belong 
granite, gneiss, schist, syenite, quartzite, slate and marble, and we 
might add anthracite coal and graphite. 

Granite, as we have shown earlier in this, treatise, was in all 
probability a sedimentary rock, such as sandstone, its materials 
derived from the sand washed from the primitive crust of the 
earth. By heat, pressure, alkali, and other chemical ingredients, 
it has been changed into the purely crystalline rock we find it, 
and without being actually fused like the lavas, it seems to have 


Ivi 


GEOLOGY OF COLORADO ORE DEPOSITS. 


been softened to such a degree that it has lost even its signs of 
stratification and appears massive. We find few evidences in 
Colorado of its being eruptive. It never occurs as a dyke pene¬ 
trating the overlying limestones or sandstones or in intrusive 
sheets amongst them as the porphyries do. 

It appears to be the bed rock of the world, underlying our 
prairies and the foundation of our mountains, and was, as we 
have said, the shore line upon which our sedimentary rocks were 
deposited. Gneiss is simply stratified granite; i. e ., granite that 
has not been so completely metamorphosed and softened as to 
entirely lose the signs of its primitive bedding and stratification. 
It is not structureless, therefore, and massive, like granite. 

Syenite is the same as granite, only hornblende takes the 
place of mica. Granite is composed of mica, quartz, and feld¬ 
spar. Syenite of hornblende, quartz and feldspar. Mica and 
hornblende schist is granite or syenite in a finely laminated con¬ 
dition, corresponding to mud shales in the unaltered rocks. 

Structureless granite is not so common generally in Colorado 
as gneiss and schist. Up the Platte canon it is finely represented 
in Dome Rock and the Cathedral Spires. The Sawatch range is 
principally granite. As a rule it is commoner in the heart of the 
mountains than near the flanks, as might be expected, since it is 
the deepest buried rock in the last condition of metamorphism. 
Its common colors are reddish from the color of feldspar, or gray 
from the predominance of mica. It is much traversed by veins 
of all kinds, principally quartz or feldspar, or both combined. 
Such combinations when there is little or no mica, are called 
pegmatite veins and it is such veins that form the matrix of most 
of our ore-bearing fissure veins in granite, such as those at 
Boulder, Central, Georgetown, etc. Granite may be distinguished 
roughly from porphyry by its non-eruptive occurrence and by its 
lacking that peculiar spotted characteristic of the porphyries. It 
always underlies, never overlies any other kind of ordinary sedi- 
mentary rock in Colorado. 

Quarzite is another metamorphic rock; it, too, was once a 
sandstone more or less pure, but of more recent age than the 
granite,as it is generally found lying near the Archaean granite, and 
belonging to some member of the Paleozoic series, such as the 
Silurian or Carboniferous. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Ivii 


Heat has changed this sandstone into a quartz-rock or quartz- 
stone very hard and usually pure white or gray, or stained with 
oxide of iron. The difference between quartz and quartzite is 
that the latter is a quartz-stone. A stone made of fragments and 
granules of the original mineral quartz, cemented together by 
liquefied quartz. 

In Colorado it may be seen forming long belts as of white 
masonry or brick walls, in our high mountain districts, lying on 
top of the granite or in strata between the limestones. As we 
retreat from the granite and rise into the upper carboniferous the 
quartzite gradually passes into hard grits and finally into common 
sandstones of the Triassic or more recent formations, among 
which few ore deposits are found. 

Quartzites may contain both gold and silver-lead deposits, 
more especially the former. They are hard rocks to mine in. 

LIMESTONES. 

Our ore bearing limestones are usually of two or three varieties 
and are mostly confined to the Silurian and Carboniferous or 
Paleozoic rocks. These are, generally speaking, in Colorado, 
magnesian limestones, commonly called dolomites. Some few are 
ordinary limestones of nearly pure carbonate of lime. They are 
mostly silicious, especially those of the Silurian period next to 
the quarzites. White nodules of chalcedonic chert or flint are 
very common in them, and give them a rough appearance where 
erosion has caused the chert to stand out on the surface. The 
Silurian limestones are characterized by white chalcedonic flints or 
cherts and by a general pale-yellow, light-gray or drab appearance. 
The carboniferous limestones above them, by a more massive 
structure, by the occurrence of black chert nodules, and by a 
dark blue-gray color. The Silurian dolomite is locally called 
“white limestone,” the carboniferous “blue limestone” for distinc¬ 
tion. At a few points in the mountains these limestones have 
been metamorphosed into marble and serpentine. 

Argentiferous lead ores are found in both these limestones, 
but the Carboniferous “blue limestone ” has been the greatest 
producer. At Aspen the blue limestone is an ordinary carbonate 
of lime or true limestone above, passing into a dolomitic or mag- 


Iviii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


nesian state below. The ore occurs penetrating both forms of 
the limestone. 

Outside of these characteristic rocks comparatively few deposits 
of importance have been found in Colorado. Hence prospecting 
has yielded little in the upper Carboniferous grits that overlie the 
productive limestone. An exception occurs, however, at Kokomo, 
where the Robinson mine has been a large producer in a belt of 
true limestone in the upper part of these grits. The region 
around Crested Butte and Irwin is also another exception, where, 
as we have said, fissure veins penetrate through all the formations 
alike. 


PREJUDICE IN FAVOR OF AND AGAINST CERTAIN ROCKS. 

There is often a prejudice amongst miners in favor of certain 
rocks and formations, and against others. Miners who have 
worked perhaps in the great Comstock mine of Nevada, or the 
Leadville mines of our own State, or the fissure veins in granite 
of the Old World, are apt to look out for and favor certain rocks 
and formations they find like those they have been accustomed 
to. Thus, as Mr. Williams says,.“ The peculiar ‘ porphyry” of the 
Comstock was hunted up in other districts, but did not prove 
metalliferous.” “ Solid granite was locked upon by others as 
unfavorable, generally, because locally some granite above the 
gold belt of California had proved barren. Yet some of our best 
veins are in granite.” 

“ Limestone was at one time a very unpopular rock and sup¬ 
posed only locally to produce lead, till the discoveries of Lead¬ 
ville and Eureka, Nevada, overturned the scale in its favor.” 

In the Leadville “excitement” not only was the particular 
Carboniferous limestone of Leadville hunted for and prospected, 
but every other limestone in the South Park region, no matter 
what its geological age or position, was extensively prospected 
without results, miners not recognizing the fact that it was not 
limestone generally that produces rich ores, but a particular lime¬ 
stone of a particular geological period (the lower Carboniferous) 
not over 200 feet thick, that happened locally to be rich near 
Leadville, and the reason of its being locally rich at that point 
was owing to the concentration of eruptive energy at that point 


GEOLOGY OF COLORADO ORE DEPOSITS. 


lix 


and the intrusion of an unusual amount of porphyries, which in 
point of fact are far more responsible for the ore than the lime¬ 
stone, which happens to be merely the receptacle. 

It was also quite common after the Leadville excitement to 
find shafts in all sorts of improbable and hopeless localities whose 
owners would tell you : “ At Leadville it didn’t matter where a 
man ‘went down ’ It was all luck whether you ‘struck it’ or not, 
and so they might as well ‘ go down ’ where they were as else¬ 
where.” It was often said “ that Leadville had exploded all so 
called scientific theories about ore being in one formation or 
locality more than another. It was all a case of luck.” 

The excuse for this is to be found in the fact that in the 
immediate vicinity of Leadville it did scarcely matter “ where 
you went down,” seeing that that area was practically underlaid 
by bedded sheets of mineral, but that such would be the case 
elsewhere and everywhere or anywhere, experience unfortunately 
has shown to be untrue. It is not a particular rock or formation, 
but a combination of favorable circumstances that alone can make 
a rich mining district. 

As experience advances geologists and miners have proved 
that ore deposits have a much wider range than was once supposed. 
Formerly only the Archaean granite series was supposed capa¬ 
ble of bearing ore deposits, because in the Old World, tin, copper 
and lead came principally from fissure veins in those rocks. Then 
deposits were found in the Paleozoic series and supposed to 
ascend no higher. But in the present day, and even in Colorado, 
they are traceable even to the Tertiary. 

It is not the rock, nor the age, but a combination of circum¬ 
stances, principally heat and metamorphism, that may make any 
rock of any period an ore-bearing one. And in prospecting in 
new regions it is these combinations rather than any particular 
rock that should be looked for. 

STRIKE AND DIP OF COLORADO VEINS. 

The dip of veins in Colorado approaches more nearly the 
vertical than the horizontal, usually from 75 ° to vertically. 
Nearly all our ore deposits, even those of the bedded class, dip 
more or less steeply from 25 0 to 75 0 . 


lx 


GEOLOGY OF COLORADO ORE DEPOSITS. 


For a few feet from the surface, on the steep slope of a moun¬ 
tain, it is common to find an ore deposit dipping quite gently or 
even folded over and dipping in a contrary direction to that 
which it assumes with depth. This appears to arise from the 
weight of the strata above it tending to bend it over downward 
in the direction of the slope of the hill. 

There is generally a prevailing dip and strike amongst a 
number of parallel fissure veins of a district. In the San Juan, 
the bulk of the fissure veins have a prevailing northeasterly 
strike and a dip to the southeast. The angle of dip is generally 
between 6o° and vertically. 

CROSS-CUTTING UNCERTAIN. 

The dip, as we have said, not unfrequently changes consid¬ 
erably with depth, usually becoming more and more vertical. 
From the degree of uncertainty as to the continuity of the dip, it 
is not always safe, on the discovery of an outcrop, to endeavor to 
cut it at a much lower point, so as to get the coveted depth, and 
better opportunities for stoping, drainage and other develop¬ 
ments of the mine. Owing to a change of dip or fault, perhaps, 
the miner may have to make a much longer cross-cut tunnel 
than he had calculated upon before striking the vein. Sometimes, 
too, he may miss the vein altogether, cutting it perhaps at some 
point where it is exceedingly thin or poor, so poor in fact that 
he passes through it without noticing it or believing it to be the 
same vein whose outcrop looked so promising on the surface. 
Cross tunnels through “ dead rock ” should hardly be under¬ 
taken until the vein has been proved to be a strong one for a con¬ 
siderable depth. As we have already shown, great depth may 
not after all be so desirable in even a fissure vein, as there is no 
certainty whatever about veins becoming richer or poorer with 
depth. Extensive cross-cut tunnels have seldom proved paying 
concerns. The greatest in the United States, the Sutro tunnel, 
six miles in length, which tapped the Comstock fissure at a 
depth of 2,000 feet, did not prove a financial success, and had it 
tapped the fissure still lower^at 3,000 feet, it would have found 
the vein in the impoverished condition it is to-day. It is not 
uncommon for a miner to strike a rich outcrop on the top of some 


GEOLOGY OF COL OF A DO OFF DEPOSITS. 


Ixi 


mountain, and on the strength of its richness induce a company 
to run a long cross-cut tunnel in “ dead rock ” half through the 
mountain to cut this vein, and the company’s resources are nearly 
exhausted in so doing, while the vein itself gives no returns, 
owing to its being left idle. Finally, perhaps, the vein is missed, 
or if struck, proves far poorer than was anticipated. Of course 
there are exceptions where cross-cut tunnels in “ dead rock ” may 
be advisable. 

If a fissure vein, as in the San Juan, should outcrop near the 
top of a mountain and be exposed on its dip all the way to the 
bottom, there may be some reason for opening a tunnel in it near 
the base, thereby facilitating drainage/development and exporta¬ 
tion. In that case the miner is on the vein, with no fear of losing 
it; but even here, there is no guarantee that it will prove rich all 
the way to its outcrop a thousand feet above. “ Follow your 
ore,” is a common and wise saying among experienced miners, 
“ and be careful how you leave it for any experimental theories.” 
We remember a tunnel in the Gunnison region which was run sev¬ 
eral hundred feet at a cost of many thousands of dollars, all through 
“ dead rock,” in the hopes of cross-cutting a certain ore body that 
had proved rich near the surface. At last it was given up, and 
subsequently a short cross-cut was made from it, and the original 
vein was found only a few feet from the tunnel, which had been 
running parallel with it all the time. The cause of the mistake 
was an unforseen fault in the vein that had shifted its dip much 
further on one side than had been calculated upon. 

STRUCTURE OF VEINS.-VEINSTONE OR GANGUE. 

In most ore deposits, whether they be called fissure veins, 
true veins, gash veins, blanket veins, or by whatever name they 
may be designated, the space between the confining “ country 
rock” or “walls” on either side is occupied by “ gangue” or 
veinstone, consisting generally of some of the elements of the 
adjacent country rock in an altered or more sparry condition than 
in the parent rock. 

The commonest of these veinstones is quartz , which is usually 
massive or of coarse or fine crystalline structure. In the San 
Juan region it is commonly a very fine-grained, hard, jaspery 


Ixii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


material, of a blue-gray color; in other places coarsely crystal¬ 
line, like loaf sugar, and frequently contains beautiful little cavi¬ 
ties called “vughs” or “geodes” or “ drusy cavities,” lined with 
long, perfect quartz crystals. In the San Juan these cavities are 
most abundant toward the center of wide mammoth veins. 

Lime is a common veinstone or gangue, particularly in lime¬ 
stone districts. It is in a white or yellowish crystalline condition, 
the crystals assuming various forms, or forming a crystalline 
mass. Crystalline dolomite also occurs in the dolomitic lime¬ 
stones of Leadville and Aspen. Stalactites of arragonite, another 
form of calcspar, are characteristic of the cavernous openings 
found associated with lead ores in limestone. 

At the Silver-islet Mine, Pitkin Co., in the Sacramento Mine, 
South Park, and in several of the Leadville and Aspen mines, 
such caverns lined with stalactites occur. These caverns appear 
to have been formed after the deposition of the ore bodies, since 
they are hollowed out of limestone and ore bodies alike, by sur¬ 
face waters, which, descending through the natural jointage planes 
in the limestone, have by the assistance of carbonic acid, enlarged 
those jointage cracks and eventuallav formed caverns, either in 
them or in the lines between the stratification planes. 

A beautiful rose-colored carbonate of manganese called rho- 
docrpsite is found in the gangue of some of the mines of San 
Juan and South Park. In South Park at one locality it is asso¬ 
ciated with quartz in a vein in granite. 

Fluor Spar , one of our softest minerals of a pale green, some¬ 
times of a purple color, occurs in fissure veins in the granite 
rock. In Bergen Park a deposit occurs of sufficient size for 
development for flux for the smelters. The white Cretaceous 
limestone of the plains has of late superseded its use. 

Baryta occurs as a veinstone associated sometimes with lime 
crystals in the limestone districts, such as Aspen, Leadville and 
South Park, where those limestones have been penetrated by 
eruptive porphyries. While veins of calcspar, common enough 
throughout the limestone rocks may or may not locally indicate 
the presence of mineral, the presence of baryta is a pretty sure 
indication of a mineral lead. It is a significant fact showing the 
relations of ore deposits to porphyry that baryta is detected as an 
element in the feldspars of certain porphyries. 


GEOLOGY OF COL OF A BO OLE DEPOSITS. 


Ixiii 


Baryta, though resembling calcspar, can be distinguished from 
it by its greater heaviness, its not effervescing with acids, its emit¬ 
ting a green flame and “decrepitating” or flying to pieces under 
the blow pipe. Its lustre is more pearly than ordinary calcite. 
As it usully occurs massive, its different crystallization is not 
always to be seen. 

Baryta is not confined to limestone regions, for we find it 
forming the gangue in several mines in the eruptive rocks of San 
Juan, notably at the Bonanza Mine on the shore of Lake Como, 
where it is associated with a good deal of gray copper. In Hall’s 
Valley it occurs in a vein in gneiss also associated with rich 
deposits of gray copper. 

It is a refractory substance in smelting processes and trouble¬ 
some if in excess. 

With these veinstones is a good deal of oxide of iron, and 
sometimes oxide of manganese, together with carbonate of cop¬ 
per stains. Carbonate of iron in the form of spathic iron or sid- 
erite very like orthoclase feldspar in appearance only heavier, 
occurs in the gangue of the Whale and Freeland Mines at Idaho 
Springs. 

The surrounding country rock generally determines the char¬ 
acter of the gangue. Thus limestone yields carbonate of lime 
for veinstone, granite its elements of quartz or feldspar, or both 
combined in what is called “pegmatite” or granulite. The por¬ 
phyries yield a clay composed of the elements of their feldspars 
and some quartz; they also seem responsible for the baryta. 

The gangue so far from being a foreign substance derived 
from unknown rocks in the bowles of the earth and filling a 
fissure through ascending solutions, is on the contrary immedi¬ 
ately derived from the adjacent country rock, and often consists 
of little more than a slight alteration or decomposition of that 
“country rock” along a crack, fissure, or other line of weakness. 
Sometimes the gangue is a porphyry dyke more or less decom¬ 
posed or highly charged with pyrites lying between walls of 
granite or some other country rock. 

DISTRIBUTION OF ORE IN THE GANGUE. 

Through these “gangues” of various characters the precious 
metal is distributed in long narrow pacthes or strings or in large 


Ixiv 


GEOLOGY OF COLORADO ORE DEPOSITS. 


crystalline masses, or in insolated or scattered crystals, or in 
decomposed masses. 

The gangue matter is generally in the majority and the ore 
thinly, sparingly and irregularly distributed in it. When a vein is 
said to be ten feet wide it is not to be supposed that ten solid feet 
of mineral from wall to wall is meant, but that that is the width of 
the gangue, while the ore body may occupy but a few inches of that 
width, or be sparsely scattered over it, the remainder being quartz 
or some other veinstone. 

It is common to find one or more rather definite and continuous 
streaks or courses of ore having a tendency to keep near one or 
the other wall, or at times to cross from wall to wall. This is 
called the “pay streak,” and is the main source of profit in the mine. 

HIGH AND LOW GRADE ORES. 

In gold veins, flakes or wires of free or native gold occur in 
the decomposed gangue, and sometimes in the pure undecom¬ 
posed .quartz. Native silver is found in the same way, but more 
as cabinet specimens than in any continuous body. Isolated 
patches of very rare or valuable minerals, such as ruby-silver, 
horn-silver or silver-glance, occur locally in parts of the vein, 
or sometimes line the stalactites or crystals of some “ vugh ” or 
drusy cavity. An assay of such picked specimens would, of 
course, give’a very unfair average of the mine. As a rule, the 
bulk of the profits of a mine are derived from the common min¬ 
erals, such as galena or pyrites, and the secondary products of 
these, such as carbonate of lead or iron oxide. It is from the 
ores, too, of comparatively low grade that the steady annual 
profits of a mine are derived. In California gold mines the aver- 
age yield of gold per ton was $16. In Dakota, $6. In the silver 
mines of Leadville the average to the ton is rarely more than 
$40, and the bulk of the ores of that richest of camps is generally 
of low grade. There are a few mines of extraordinary high 
grade in sufficient quantities to yield from $75 to $100 per ton, 
such as some of the^mines of Aspen, but these are exceptions 
rather than rules, and even these have a large quantity of low 
grade ore on hand. Quantity of ore and facilities for milling, and 
the size of the vein and its facility for work, or its nearness to 
market, give the offset. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Ixv 


DECOMPOSED ORES. 

Sometimes the gangue matter contains a variety of decomposed 
ores in rich secondary combinations intimately mixed through its 
mass, and rarely discernable by the naked eye. Thus a mass of 
seemingly yellow mud may be found by assay to run high in 
silver from the concealed presence of chlorides or sulphurets of 
silver in it. No accurate estimate of the value of a mine or even 
of a piece of ore can be formed without an assay or a mill run 
being made. A vein may appear sparkling with masses of galena 
or glittering with golden pyrites, and seem to the unexperienced 
a perfect bonanza of wealth. The assay or mill run value may 
show the galena to be very poor in silver or the pyrites to yield 
no gold. On the other hand a mass of heavy rusty dirt may 
assay up into the thousands. The reason for such richness in 
decomposed surface products appears to be that nature has been 
for ages leaching out, concentrating and combining in richer 
forms the essence, so to speak, of the vein. A rich mine in the 
San Juan ships nothing but yellow mud, and another, the National 
Belle, at Red Mountain, yielded similar materal when we visited 
it, which had to be dried before shipping, but gave steady and 
good returns. It is evident then, how unsafe it is to judge of a 
mine by the sight alone. Truly “all is not gold that glitters/’ and 
the necessity of a thorough assay or mill-run when possible, of 
portions of every accessible part of the vein, and especially of 
those poorer portions which yield the daily average, is obvious 
before the real value of a mine can be estimated. 

GRAY COPPER. 

Besides the ordinary galena and pyrites we often find con¬ 
siderable bodies of gray copper intermingled with other ores. 
This is nearly always a rich ore, varying, however, in different 
localities from 60 oz. silver to several thousands per ton. It 
occurs generally in the massive state, rarely showing its pyramidal 
“tetrahedrite” crystals. 

In appearance it is not unlike a freshly broken piece of bronze, 
tin or iron. It is more common in the fissure veins in the granite 
and eruptive rocks than in limestone deposits. In Hall’s Valley 


Ixvi 


GEOLOGY OF COLORADO ORE DEPOSITS . 


it is associated with baryta in a vein in the gneiss. It occurs in 
the Georgetown veins in granite. In the San Juan district it 
occurs also associated with baryta in the Bonanza mine, and an 
ore not identical with it in composition, but very like it in appear¬ 
ance, called bismuthinite, consisting of bismuth, antimony, copper 
and silver, is characteristic of that region and is rich in silver. 
Bismuthinite has a more shiny tin-lke appearance than gray 
copper, and the red color which bismuth gives to charcoal under 
the blow-pipe readily distinguishes it from gray copper. 

LOCAL VARIATIONS IN VALUE OF ORES. 

There are locally in different mining districts considerable 
differences in the value of certain minerals and ores. In one dis¬ 
trict gray copper may rarely exceed 60 ounces of silver, in 
another it is invariably over ioo ounces. 

A coarse galena is generally poor in silver, while fine grained 
“ steel galena ” is generally rich in silver, but the reverse may 
also be the case. In some of the mines at Aspen fine grained 
galena, especially near the surface, is quite poor in silver, while 
in other mines in the same district it is exceedingly rich. Locali¬ 
ties occur also where coarse grained galena runs well in silver 
and is. richer than fine-grained galena. This is the case at the 
Colonel Sellers mine at Leadville. So one mining district or 
even one mine is not a rule for another. 

PYRITES. 

Iron pyrites and copper pyrites, common in most of our 
quartz veins in granite and in the eruptive rocks, may yield both 
gold and silver, but usually the former. There are certain dis¬ 
tricts more characterized by pyrites than others, such as the 
Central City district. These are generally gold-producing dis¬ 
tricts. Some of the mines of Breckenridge and South Park have 
strong pyritiferous veins in eruptive dykes, such as the Jumbo 
mine. These have of late produced a great deal of gold. The 
same district, however, produces large argentiferous lead veins. 
Pyrites generally favor the granite, eruptive and crystallized rocks. 
The quartzite of the lower Silurian of South Park and Redcliff 
are often pyritiferous and generally gold-bearing. In limestone 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Ixvii 


the pyrites is rare or absent, its place being filled by some form 
of iron oxide. In the deeper mines of Leadville, however, this 
iron oxide is beginning to pass down into the iron sulphide or 
pyrite from which it was derived. Iron pyrites can generally be 
distinguished from copper pyrites'by its paler, more brassy color, 
by its superior hardness and by its crystallizing in cubes. Copper 
pyrites much yellower and softer, and crystallizes in a more 
pyramidal form. A vein may glitter with showy pyrites and yet 
be quite valueless. It usually yields more gold in its decom¬ 
posed, oxidized condition than in its unaltered state. In the one 
case the gold is free-milling, and in the other it must be smelted 
at much greater expense. 


“ SULPHURETS.” 

This term amongst miners is loosely used, and often means 
some decomposed ore whose ingredients cannot be determined 
at sight, but which somehow assays high in silver. True sul- 
phuret or sulphide of silver is a name embracing a large family 
of rich silver ores, among which are stephanite or brittle silver, 
argentite or silver glance, sylvanite or graphic tellurium, and 
polybasite. 

All these rich ores are compounds of sulphur and silver and 
other ingredients in varying proportions. They are somewhat 
alike in appearance and not always so easy to distinguish. 

Argentite , silver glance, or sulphuret of silver, is of a black¬ 
ish, lead-gray color, easily cut with a knife, and consists of an 
aggregate of minute crystals. Its composition in 100 parts is 
sulphur 12.9, silver 87.1. Under the blow-pipe it gives off an 
odor of sulphur, and yields a globule of silver. 

Stephanite , or “ brittle” or “ black ” silver, is closely allied to 
argentite. Its composition is sulphur, antimony and silver, silver 
being 68.5 per cent. The crystals are small. Under the blow¬ 
pipe it gives off garlic fumes of antimony. Yields a dark globule 
from which, by adding soda, we get pure silver. 

Polybasite , common at Georgetown and in some of the Aspen 
mines, such as the Regent or J. C. Johnson, on Smuggler Hill, is 
like the others, but of a more flaky, scaly and graphitic appear¬ 
ance. It is not unlike very fine-grained galena, yielding 1 50 to 
400 ounces of silver per ton. 


Ixviii 


GEOLOGY. OF COLORADO ORE DEPOSITS. 


These sulphurets sometimes line little cavities in limestones 
with a dark sooty substance which under the microscope prove 
to be crystals of one of the sulphurets of silver. Sometimes also 
a rock is stained all through a blackish gray by these sulphurets. 
Iron or manganese may produce much the same effect, but an 
assay will soon reveal the difference. Associated with such a 
rock we may see flakes or wires of native silver that have emerged 
from the sulphide state. 


CHLORIDES. 

Chloride of silver, “horn silver” or cerargyrite. This is 
another result of secondary decomposition from a sulphide state, 
(silver sulphide.) It is a greenish or yellowish mineral, like wax 
and easily cut with a knife, it is a very rich ore running 75.3 per 
cent, silver, the remainder being chlorine. As a secondary pro¬ 
duct of composition it is generally found near the surface or in 
cavities, sometimes deposited on calcite or other crystals. In the 
mines of Leadville it is commonly associated with other decom¬ 
posed ores such as carbonates. In the Chrysolite mine, a mass 
weighing several hundred pounds was found. Chloride, bromide 
and iodide of silver are closely related, being compounds of chlor¬ 
ine, bromine, iodine, and silver. It is noticeable that these salts 
are the elements of sea water, and that these ores are often found 
in marine limestones. According to Mr. Emmons the change at 
Leadville from sulphide to chloride was produced by surface 
waters; these waters are found to contain chlorine, which thev 
probably derived from passing through the dolomitic limestones 
which contain chlorine in their crystals and these limestones per¬ 
haps originally derived it from the sea water in which they were 
deposited. Chloride of silver is found at Aspen and abundantly 
in the out-crop of mines in New and Old Mexico. 

SULPHARSENITES. 

Ruby silver , (pyrargyrite and proustite.) Composed of sulphur 
17.7, antimony 22.5, silver 59.8=100. Crystallizes in rhombohe- 
drons is seen in spots or crystals on a mass of ore of a deep red 
or blackish tint. When scratched with a knife it shows a bright 

o 

or deep red color. In some mines this very rich ore occurs only 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Ixix 


as specimens, but in others it is present in sufficient quantity to 
largely influence the value of the ore in bulk. In parts of the 
Granite Mountain Mine in Montana it constitutes the principal 
ore, associated, however, with other mineral. It there occurs in 
large masses and accounts for the extraordinary richness of that 
celebrated mine. Proustite is much the same only lighter red 
and consists of sulphur 19.4, arsenic 15.1, silver 65.5 = 100. 

CARBONATES. 

This term also embraces a large family, the commonest being 
carbonate of lead, (cerussite) and carbonate of copper, (malachite 
and azurite.) 

Copper carbonate can never be mistaken owing to its brilliant 
green and azure blue color. Copper stains are among the com¬ 
mon surface signs of a “lead.” It is generally associated also 
with rusty stains. Both are the surface products from copper and 
iron pyrites forming a vein below ground which may or may not 
be profitable. Copper stains are common enough in many rocks, 
but do not always lead to bodies of ore. In South Park the red 
Triassic sandstones are so stained, but yield no ore. Along our 
foothills there is quite a stained belt from Golden to Morrison 
and through Bergen Park. But few promising deposits of copper 
or other ores have been found, although handsome specimens of 
native copper have been discovered near Golden. 

At the Malachite mine on Bear Creek, near Morrison, a pros¬ 
pect was at one time opened showing a good deal of silicate of 
copper (chrysocolla), and malachite, but for some reason it has 
not been worked since. 

Copper in its native or uncombined state is rare in this State, 
and so far we have as yet no true profitable mine. A great deal 
of copper is found.associated with other ores, and is extracted by 
some of the smelters. Carbonate of copper is commonest in the 
limestone districts as might be expected from the carbonating 
influence of limestone upon minerals in it, or mineral solutions 
passing through it. Carbonate of iron (spathic iron, or siderite), 
constitutes part of the gangue matter in some of our veins, and 
may also be found associated with coal seams generally, in the 
latter case in an oxidized condition. 


Ixx 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Carbonate of lead (cerussite). This is mostly found in the 
limestone districts, such as Leadville. It is there known in two 
forms, one called “hard carbonates,’’ the other “soft” or “sand 
carbonates.” The crystals of this ore are small prisms, some¬ 
times combined into a cross shape, of a pale grayish white, and 
might be taken for some form of carbonate of lime or gypsum, their 
weight, however, soon shows the difference. They are a secondary 
product of decomposition consisting of carbon dioxide and lead 
oxide, as a carbonate they effervesce in nitric acid, and yield lead 
when heated. Cerussite is exceedingly rich in lead, carrying 75 
per cent The white lead of commerce has the same composition. 
In Leadville and elsewhere in Colorado it is silver-bearing also, 
and though low in silver, the facility of its treatment at the 
smelter makes it a very desirable ore. As a rule it contains less 
silver than the unaltered galena, but is more easily treated than 
the latter. The process of change or derivation from a sulphide 
state (i. e ., from galena) to a carbonate, is well shown sometimes 
in a piece of Leadville ore. A central cube of galena is surrounded 
by a grayish green ring of sulphide of lead or anglesite, and outside 
this may again occur crystals of lead carbonate. Thus the process 
is from a sulphide to a sulphate, then to a carbonate. The so 
called “hard carbonates” is a brown mass consisting of a hard 
flinty combination of iron oxide and silica, impregnated with crys¬ 
tals of lead carbonate, with which are often silver chlorides, also. 
The “sand carbonates” result from the decomposition and breaking 
up of the hard carbonates, or from a mass of pure crystals of 
carbonate of lead, which are, by nature, loose and incoherent. 
The Leadville mines are getting below these products of decom¬ 
position and entering upon the original sulphides of galena and 
iron. The yield, however, is said to be equally good. 

ZINC-BLENDE (SPHALERITE), “BLACK JACK.” 

Common in most mines mixed with other ores. As it is a 
very refractory mineral in smelting, much of it is not desirable in a 
mine. 

It is easily recognized by its brown resinous look or when 
very black by its pearly lustre. At Georgetown, near the surface, 
brown “ rozin zinc-blende ” carries silver, and is associated 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Ixxi 


with rich ores, such as polybasite and gray copper. With depth the 
zinc-blende becomes more abundant and blacker, and loses much 
of its silver properties. Zinc-blende may run from nothing to 
twenty dollars silver, and rarely as high as $100 per ton. 

In some mines in the San Juan it occurs abundantly near the 
surface and fades out with depth. W r e have no true zinc mines in 
this State, the zinc being mixed with other ores. In some mines 
in Pitkin County the zinc predominates over all other ores, and 
though it runs high in silver the smelters do not care to take it, 
on account of its refractory character. In the Eastern States 
where zinc smelting is a specialty such ore might be separated 
and both silver and zinc saved. 

In Colorado there are no mines of one mineral alone, as in 
some other parts of the world. We have no true lead, zinc or 
copper mines; these baser metals are either argentiferous or 
auriferous, and their baser qualities are sacrificed for their richer 
ones. 

“ BRECCIAS ” AND “ HORSES.” 

Some lodes enclose fragments of the country rock. Wdien 
these are small and angular the vein is said to be “ brecciated 
when they are large so as to split the vein they are called by 
miners “ horses.” 

Breccia usually occurs in large veins near the walls. This is 
frequently seen in the fissure veins of the San Juan region. Frag¬ 
ments of the adjacent eruptive rock are enclosed by a bluish 
quartz. 

In the same region, where we have extraordinary opportuni¬ 
ties of seeing complete sections of great fissure veins descending 
the face of a cliff for 2,000 feet, on either side of a profound canon, 
a broad vein is at intervals observed to split up into two or three 
arms enclosing large fragments of the “ country rock,” and these 
are then seen to unite again in a broad vein, which at another 
interval will again include a large “ horse,” or split up into a 
number of branches, which again unite to form the main vein. 

These veins occupy a once shattered fissure, the walls of 
which are neither straight nor regular, but shattered either into 
small fragments near the vein, or into large ones extending into 
the country rock. The vein material has insinuated itself in 


Ixxii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


between the shattered portions, sometimes enclosing small frag¬ 
ments and producing a “ breccia; ” at others around large frag¬ 
ments, producing a “ horse.” Sometimes the brecciated fragments 
are surrounded by rings of ore, and are called “ cockade ores.” 

SUBSTITUTION BY SILICA. 

Quartz substitutes itself for mineral elements or portions of 
the country rock, as matter in a wounded limb substitutes itself 
in the place of the natural flesh. Rocks have been wounded by 
narrow cracks; these cracks have been healed by silicious matter 
oozing or supplied from the adjacent rocks in solutions ; the solu¬ 
tion matter not only heals the crack, but eats into and substitutes 
portions of the country rock on either side the original fissure 
with its own matter; and so, we think, are sometimes formed 
wide fissure veins of quartz, the original crack or fissure being 
perhaps not an inch in width, but simply a line of weakness for 
mineral solutions to work upon and exercise substitution. This 
substitution or metasomatic replacement is not unlike that which 
takes place in the fossilization of a tree-trunk. A tree falls into 
a marsh, is buried in mud and sealed from contact with the air. 
It is soaked with water carrying silica in solution ; as the cells of 
woody fibre gradually rot and pass away each molecule of wood 
is replaced by a microscopic molecule of silica. The result is a 
tree substituted or replaced by silica, or as we say, a silici- 
fied, fossilized trunk. So breccias may be sometimes formed by 
quartz substituting itself for portions of the country rock, leaving 
harder or unchanged portions not yet substituted still remaining 
in the vein surrounded by the quartz. Fragments forming brec¬ 
cias do not seem to have fallen into a wide, gaping fissure, gradu¬ 
ally filling up with quartz in solution, and so got entangled in the 
quartz, for many of the pieces will match one another, and appear 
rather as if quartz in solution had enlarged slight cracks between 
the fragments, or else, as we have said, substituted a large por¬ 
tion of the country rock, leaving portions still unsubstituted. 
“ Enclosed fragments of country rock are nearly replaced by 
silica, leaving only a shadowy image of their original forms.” 

“ BANDED, COMBED OR RIBBON ” STRUCTURE. 

“ The various minerals filling a vein fissure are frequently 
arranged in a succession of bands parallel to the walls. These 



■F/g J Morses in great Fissure Veins 

-4NIMA 5 B. IVOR L AM DM . 5'A Afc/tfA M CoL/J 

j 4 . pusiMHrtsVe/n/j B.Morse; C: Corel. J) 7'unneJj 



CF/lB Morses an d Split Veins 
Animas B/ven Ca non , 5’anJnam FoJd. 
-A . Prs/ru-viirt/y Tu. "taels, Vj, Byron lLn»el- 



Mg 3 Itrtjii/feraus Ve/ns expo serf to v)ew 

vox A ffoWAAVS VJL CO _ £ AN J UA N FOL O . 
JAoreirtaioy<fte/nJr//AsureM//n / Ctoan/io or/e unottoi 





















































































V 






















-• ' • > 

















GEOLOGY OF COL OF A E>0 OFF DEPOSITS. 


Ixxiii 


bands or ‘ combs ’ are aggregations of crystalline minerals, the 
separate crystals of which are usually arranged with their longer 
axes at right angles to the walls of the lode, while their crystal¬ 
line form is more perfectly developed at the ends turned towards 
its center than at the other extremity.” These combs meeting, 
if their crystals happen to be long pyramids of quartz, remind 
one of two rows of teeth clenching and interlocking. 

“ This ‘ ribbon ’ or banded structure indicates long-continued 
chemical action, occasionally interrupted, but again renewed 
under different conditions, the substances deposited in the walls 
varying with the nature of the minerals held in solution at the 
time the bands were severally formed. Some parts of a vein may 
show this comby structure, while others show no trace of any 
particular arrangement.” Perhaps the latter case might be due 
to greater pinching or pressure, not admitting of freedom for 
crystallization. Toward the center of large quartz veins, “ vughs,” 
or cavities of a lenticular shape occur, lined with beautiful quartz 
crystals, their points pointing to the center of the cavity, at right 
angles to the wall There seems to have been an open space 
here, or a relief of pressure sufficient to allow the quartz to crys¬ 
tallize out in such large and perfect forms. If the vein was filled 
by “ lateral secretion,” i. e ,, by solutions coming from the sides 
and forming on either wall, there would be naturally a space in 
the middle. In the basaltic rocks of Table Mountain, at Golden 
City, cavities have been formed in the molten lava by steam bub¬ 
bles. These cavities were subsequently lined by a succession of 
zeolite crystals, first chabasite, on this thompsonite, and lastly 
analcite. The minerals for these crystals were derived by solu¬ 
tions from the basalt in different degrees of combination, and so 
crystallizing in different forms. This is probably analogous to 
the vein ribbon structure. “ Fortification ” and ribbon agates are 
of the same nature. 

Vein fissures, being a line of weakness, have occasionally 
been re-opened and a new or different character of veinstone or 
mineral formed in the interstice, giving rise to a series of alter¬ 
nate “ ribbons.” 

The re-opening of a fissure has been attended by a grinding 
together of the walls, resulting in slickensides and gouge, and 


Ixxiv 


GEOLOGY OF COLORADO ORE DEPOSITS. 


this may account for the appearance of gouge in the center or 
heart of a vein which has been re opened. 

The arrangement of the minerals in these alternate bands or 
ribbons could only be produced by solution or sublimation. The 
successive layers are produced by deposits parallel to the walls, 
while the crystals have their axes directed to the center of the 
vein. This banded, ribbon, combed and “ vugh ” structure is 
common in the San Juan district. 

“ In vein fissures,” says Phillips, “ formed at different periods, 
the mineral deposits will depend upon the nature of the sub¬ 
stances dissolved in the waters circulating through them at th6 
time of their formation. In the same locality the nature of these 
solutions has changed from time to time, hence the variety of 
minerals found at intervals in a mine. In nearly all wet mines 
secondary minerals of various kinds are in progress of formation 
from the original sulphides of the vein proper.” Galena that has 
been left lying on a dump for many years has been known to 
pass gradually into a sulphate and thence into a carbonate of lead. 

CHANGE OF MINERALS WITH DEPTH. 

Lodes often change in the character of their minerals with 
depth, not only after they have left the zone of secondary decom¬ 
position and surface action, but also far below it. Thus, in the San 
Juan some of the mines abound in zinc-blende near the surface, 
which with depth almost disappears, giving place to gray copper 
and other superior ores. In Cornwall, England, the shallow 
workings yield copper, and with depth, tin ; and locally, many 
such changes may characterize a particular district, but cannot be 
formulated as a rule for other localities. 

INFLUENCE OF COUNTRY ROCK. 

In most mining regions, to which Colorado is no exception, 
a relation has been observed between varieties of “country rock” 
and ore deposits. Veins in passing from one country rock to 
another are liable to change in the size or variety of the ore, 
widening in connection with some rocks, and pinching or grow¬ 
ing narrower in connection with others. 

Certain rocks are notorious ore-bearers, whilst others are 
notoriously barren over large regions, or in special localities. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Ixx V 


The presence of certain rocks adjacent to other different rocks 
has an enriching tendency on the ore bodies. 

As regards rocks that are good ore-carriers or receptacles of 
particular classes of ore in Colorado, we may say that quartzites 
and silicious rocks generally carry more pyrites, and are gold- 
bearing. 

That veins in granitic rocks carry a greater variety of min¬ 
erals than others, and may be both gold and silver-bearing. 

That certain limestones carry much argentiferous galena. 

That sandstones and other unaltered rocks carry little ore of 
any kind. 

The influence of country rock on veins may be from several 
different causes, for instance : 

Certain rocks are by their structure better adapted than others 
for forming regular fissures. Thus, massive limestone is better 
fissured than slate or shale, leaving wider open spaces for the ore 
to collect in. 

Other rocks may be more porous, and admit mineral solutions 
through their pores. Of such a kind are some of our porphyries. 

Others, like limestone, are easily acted upon by solutions dis¬ 
solving out the rock and replacing it with mineral by substitu¬ 
tion. 

Some are better conductors of heat, and therefore would assist 
chemical action and mineral solution. 

And lastly, if modern theories of “ lateral secretion” be true, 
viz., that most ore comes from the adjacent country rock and is 
precipitated, substituted, or collected in the vein fissure, and fur¬ 
ther, that the metals themselves are derived from certain metallic 
elements in the ordinary constituent minerals of the country rock, 
such as mica, hornblende, or augite, it is clear that a rock com¬ 
posed largely of such minerals would be liable to influence the 
vein as an ore generator. Granite and porphyries are largely 
composed of these minerals. 

The frequent presence of eruptive porphyry rocks near veins 
and ore deposits in Colorado shows that they have an important 
influence on those deposits, which may be of various kinds. 

First, that in their component minerals and mass they actually 
contain the elements of the precious metals subsequently depos- 


Ixxvi GEOLOGY OF COL OF A E>0 ORE DEPOSITS. 

ited in another form in the fissure vein or in the soluble limestone 
in contact with it. 

Secondly, by the heat which they retain for a long time after 
they have congealed and hardened, they would assist in the reac¬ 
tions of any chemical or mineral solutions that might be on hand. 
Lava, at the time of its eruption, is always highly charged with 
steam and other gases. By reason, also, of the chemical compo¬ 
sition of porphyry, waters passing through it would be alkaline 
and assist in dissolving silica and other gangue or veinstone mat¬ 
ter, and when the porphyry has thoroughly cooled it is exceed- 
ingly porous, and being much jointed and cross-fractured, becomes 
like a great sponge for the absorption of all surface waters. This 
may be noticed at Aspen, where all the mines that are at present 
penetrating through the “porphyry cap” are much troubled with 
water, far more so than in the underlying limestone. Surface 
waters, then, becoming alkaline by passing through this rock, 
and also more or less charged with carbonic acid, chlorine, and 
other solvents, would be ready to dissolve both gangue and vein 
ingredients out of the porphyry and redeposit them in the vein 
fissure, or, by metasomatic substitution, in the limestone usually 
beneath it. 

Water circulating in fissures changes or dissolves the ingre¬ 
dients of the surrounding rock. The rocks enclosing lodes are 
always so altered, and this decomposition and alteration is not 
always merely local or confined to the close proximity of the ore 
body, but we often find a whole mining district, such as Lead- 
ville, Aspen and San Juan, pervaded by this feature. So much 
is this the case that it is often difficult to get a fresh, unaltered 
specimen of porphyry or some other country rock within the 
district. 

The brilliant red, yellow and maroon tints that color so much 
of the mining district of San Juan result from the oxidation of 
pyrites and other iron-bearing minerals pervading the eruptive 
rocks, and it is noticeable that this color, resulting from alteration 
and decomposition, is most prominent in those parts where lodes 
have been discovered, as, for example, the gorgeous tints of the 
Red Mountain area around the celebrated “ National Belle,” 
“ Yankee Girl,” and Ironton mines, between Silverton and Ouray. 


GEOLOGY OF COL OF A DO OFF DEPOSITS. 


Ixxvii 


The rocks in Geneva Gulch, Hall’s Valley, Buckskin Canon, and 
in other mining centers, display the same beautiful tints of oxida¬ 
tion in the vicinity of the mines. 

“ In lodes a mutual exchange takes place through the reaction 
of the ingredients of the rock and the materials of the vein. 
Thus, when water containing carbonates comes in contact with 
rocks or minerals containing alkalies, a chemical reaction takes 
place. When these last are combined with silicic acid, these 
silicates are decomposed by the carbonic acid and the bicarbon¬ 
ates. This explains both the crystallizing out of the carbonates 
and the so frequent decomposition of rocks containing lodes, 
especially those which, like our veins of granite, are feldspathic.” 

The same principle applies to other ores and minerals in 
lodes. Thus the precious metals in tile mines of Leadville in 
their original condition have been proved by depth to have been 
in a sulphide state, such as iron pyrites (sulphide of iron), or 
galena (sulphide of lead), etc. Surface waters charged with car¬ 
bonic and other acids, passing through the overlying porous 
alkaline porphyry and entering the underlying limestones, have, 
as we have previously observed, changed the sulphides into sul¬ 
phates, oxides and carbonates. 

The presence of a dyke near to or cutting a vein has been 
found often to enrich the latter at the point of contact. 

In the “ Colorado Central ” mine at Georgetown a narrow 
dyke of brown obsidian traverses a larger dyke of ore-bearing 
porphyry. The valuable ore is found close to the obsidian dyke. 
This might be the result of greater heat at that point. The 
“ black dyke ” in the Comstock mine is a somewhat similar case. 

PALEONTOLOGY OF ORE DEPOSITS. 

By finding certain characteristic fossils in the qountry rock 
enclosing the ore bodies, we determine its geological age. Thus 
in Colorado, in the Leadville region, we find occasionally in the 
ore-bearing “blue limestone” a sort of pectinated cockle-shell with 
a broad groove down the middle of the shell; this is called a Spiri- 
fer (Spirifer Rocky-Montana). Another rather hump-backed, 
round-shouldered shell, the size of a walnut, with short spines 
sticking out from it, is called a Productus ; and a third, of a spiral 


Ixxviii GEOLOGY OF COLORADO ORE DEPOSITS. 

shape, not unlike a large snail shell, is called Euomphalus, and 
another Pleuroto-Maria. These shells being characteristic of 
and peculiar to the Lower Carboniferous in various parts of the 
world, label the geological horizon whose rocks carry the ore at 
Leadville as belonging to that period. 

At Aspen, some seventy-five miles distant, the same fossils 
occur in a “blue limestone,” together with a great number of fossil 
corals, some cup-shaped, and others full of pores like the section 
of a sugar-cane. The former belong to the “ Zaphrentis ” class 
of corals ; the latter are called “ Syringopora.” 

The similarity between the fossils in the ore-bearing blue 
limestone at Leadville and Aspen enables us to state that the ore 
deposits of both regions are in the same geological horizon and 
the ore bodies situated in very nearly the same belt and much 
the same circumstances. 

The importance of this is obvious. Leadville having pro¬ 
duced so wonderfully in the past, if it can be proved that the new 
camp of Aspen has deposits in the same belt and similarly situ¬ 
ated it would be so much in Aspen’s favor. 

Too much reliance as to the future of any camp must not be 
grounded on these geological facts. Local circumstances, besides 
those of merely being on the same geological horizon, may 
have much to do with the distribution of ore. Thus, while it 
would be wise to follow up this particular limestone, and prospect 
it wherever it appears between Leadville and Aspen, it by no 
means follows that it will everywhere prove productive of mineral. 
The most likely places for mineral deposit would be where it 
happens locally to be traversed by eruptive porphyries, or where 
there are signs of porphyries or eruptive rocks being in the vicin¬ 
ity of the limestone. 

As limestones of different ages are often very similar to one 
another in color and composition, the finding of these peculiar 
fossils is very useful in tracing them over the country; but in 
Colorado these fossils are not so common as we might wish, and 
in lack of them a prospector might form a shrewd but not very 
certain guess at the identity of this ore-bearing limestone by its 
position on a cliff section relative to other formations. 

Generally speaking, this Lower Carboniferous limestone may 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Ixxix 


be found four hundred to five hundred feet above the granite, the 
interval being occupied by a well-defined belt of white quartzite 
about 200 feet thick, and upon that a drab-yellow dolomitic lime¬ 
stone (“white limestone’’*), also about 200 feet in thickness. Of 
course this thickness may vary much witii localities. Then 
again, above the blue limestone usually comes a bed of black 
carbonaceous shales, then a thick bed of grits and sandstones, and 
then red sandstones. The blue limestone will lie then somewhere 
between the red sandstones and the granite. The long section 
accompanying this report will show the general position of these 
strata. 

A lack of ability to distinguish the age and position of cer¬ 
tain limestones has lead to a great deal of useless prospecting in 
Colorado. On the other hand, largely by means of these fossil 
shells, the Geological Survey has been able to follow the Lead- 
ville ore-bearing belt for many miles along the Mosquito range 
and over the range to the region around Mt. Lincoln, Brecken- 
ridge and South Park, and again further North up the Arkansas 
river to the Eagle river, to Redcliff, and thence to Aspen. Simi¬ 
lar rocks, too, have been identified by means of their fossils, 
as far down as Tombstone, in Arizona, and Silver City, New 
Mexico. 

To resume our section : Next below the blue limestone, with 
sometimes an intervening bed of white quartzite, called the “part¬ 
ing quartzite,’’ comes a bed of 200 feet or more of drab-yellowish 
dolomitic limestone, full of white flints; it is locally called “white 
limestone.” In this a few shells have been found sufficient to 
identify it as belonging to the Silurian. This limestone yields 
ore, but not generally so well as the “ blue lime.” 

Below this is a bed of hard, white, crystalline quartzite, which 
has yielded very few obscure fossil impressions. The fossils 
appear to have been obliterated by the heat attending metamorph¬ 
ism. From its general position on top of and immediately upon 
the Archaean granite, and its position underneath the fossil- 
defined Silurian and Carboniferous above, it is considered to 
belong to the lowest division of the Silurian, called the Cambrian. 
This Cambrian quartzite has yielded gold in South Park and at 
Redcliff. 


* Known also as “short lime ” by Aspen miners. 



Ixxx 


GEOLOGY OF COLORADO ORE DEPOSITS. 


This brings us to the granite, in which there are no fossils, 
but it is not difficult to recognize it. It is the “ bed rock ” of the 
world, the Archaean or beginning, and here paleontology stops. 

If we now begin again from the top of the Carboniferous 
limestone and ascend, we shall find dark shales, stained with 
coaly material, and even some small seams of anthracite coal, and 
among them (near Sheep Mountain, in South Park), actual 
impressions of the singular foliage of the Carboniferous epoch, 
such as gigantic horsetail rushes (equiseta), and treeTerns sim¬ 
ilar to those found overlying the coal in Pennsylvania. 

These fossils confirm our opinion as to the ore-bearing lime¬ 
stone being Lower Carboniferous, and that these grits and shales 
represent the Middle Carboniferous. The grits have not, so far, 
produced much ore. Above this thick bed of grits, called Weber 
grits because they are developed on an enormous scale in the 
Weber canon in Utah, we find a few beds of limestone, some 
shales and some coarse, dark-brownish red conglomerates. 

The limestone has yielded some fossil shells, showing it to be 
Upper Carboniferous, and at Kokomo, in Ten-Mile district, at the 
Robinson mine, large bodies of ore have been discovered asso¬ 
ciated with eruptive porphyries. This is, with the exception of 
the ore deposit in the Gunnison district, about the highest horizon 
in which ore bodies are found in place in Colorado. 

[In the shales near P'airplay, South Park, the writer discov¬ 
ered some interesting remains of fossil insects and foliage, which 
have been determined as belonging to the Upper Carboniferous.] 

Next follows a great thickness (from a thousand to two thous¬ 
and feet) of coarse red conglomerate sandstones, supposed from 
their position relative to well determined Carboniferous beds 
below, and Jurassic beds above, to be the Triassic and to mark the 
beginning of the Mesozoic era in Colorado. No fossils have been 
found, however, to make this certain. A few copper stains is all 
the mineral found in this formation in this State. 

Next above this are a series of variegated clays and shales, 
red, purple, green and gray, also a bed of thick white sandstone 
near the base which has been used for glass. Being nearly pure 
silica. There is a bed of limestone in the middle which is burnt 
for lime among the foothills. In the shales and clays of the upper 


i a Miai m 



Foldout Placeholder 

This foldout is being digitized and will be 
inserted at a future date. 





















‘V ■ {.V Ail] 

,, > 

' ' .k . 






r 


MAAHO'A/; 


















GEOLOGY OF COLORADO ORE DEPOSITS . 


Ixxxi 


portion of this Jurassic formation the writer discovered some large 
skeletons of dinosaurs or “terrible land lizards” at Morrison on the 
foothills and also in Wyoming. These remarkable fossils show 
the formation to belong to the Jurassic. Some shells discovered 
in the limestone in Wyoming also prove the same thing. There 
are no important minerals in this group. Oil has been found in 
the saurian beds in Oil Creek, near Canon City. Above this, 
along the foothills and in the basin of the parks, is the prominent 
sandstone “hogback” of the Dakota group, the floor of the Cre¬ 
taceous epoch, identified by peculiar leaf fossils found in it. In 
the center of this “hogback” is found a bed of fire-clay of excel¬ 
lent quality. 

The middle or Colorado group of the Cretaceous consists of 
dark and drab clays full of marine shells, some of them called 
Ammonites are often mistaken for fossil snakes coiled up. An¬ 
other shell, often covered with mother of pearl resembles an eel 
cut in two, and is often supposed to be a fossil fish. In the mid¬ 
dle of this Colorado group, between one and two thousand feet 
below the coal beds, oil has been found in a loose sandstone at 
Florence, near Canon City. There appears to be two horizons 
or two oil sands near Canon City, one in the upper Jurassic, the 
other in the Colorado group of the Cretaceous, both may probably 
originate from strata of a still lower geological horizon. 

Near the base of this Colorado group is a prominent bed of 
white limestone, much quarried for flux for the smelters, being a 
nearly pure carbonate of lime. In it are found large round shells 
about the size of a saucer, and not unlike a modern clam ; these 
are called “inoceramus.” This limestone near Schofield, on Rock 
Creek in the Gunnison, is traversed by argentiferous galena veins. 

Next come the Laramie Cretaceous coal beds consisting of 
sandstones, clays and shales. The important coal seams lie in a 
thick bed of sandstones near the base of this group. Beds of 
inferior limonite iron ore are also found a short distance above 
the coal. The upper portions of this group for 1,000 feet have 
yielded, at intervals, very good artesian water, at Denver and else¬ 
where. Throughout the Laramie group abundant fossil remains 
of tropical foliage, such as palm leaves, fig leaves, cinnamon, etc., 
have been found to identify its horizon. The sandstones near the 


Ixxxii 


GEOLOGY OF COLORADO ORE DEPOSITS. 

coal are quarried for building stone. The Tertiary which lies on 
the top of this, and unconformably with it, consists of pudding- 
stone, conglomerates, shales, thin seams of coal and plant remains. 
Its unconformity to the Laramie is among the leading characteris¬ 
tics that prove its Tertiary origin. It generally forms table lands, 
and is often capped by some form of more recent lava, the latter is 
quarried for building stone. Spread over all these formations 
alike, in mountain canons and along river courses, we find the 
glacial moraines, cobble stones, sands and boulders, sometimes 
modified by water into stratified beds. These, as we have said, 
are our gold placer beds. Remains of the mammoth and mas¬ 
todon have been found in these beds in Colorado, a sufficient 
evidence of their Glacial and Quaternary origin. 

On top of this is the soil of the present age, in which we 
find bones of buffaloes and deer, and relics of civilized man and 
uncivilized Indians. 

It will appear from the colored section to what a limited 
portion of this ten to twenty thousand feet of strata our precious 
metals are restricited, viz.: To the first few hundred feet of the 
lower and geologically older portion. 

GEOLOGICAL AGE OF VEINS AND ORE DEPOSITS. 

As veins are necessarily of more recent date than the rocks 
which contain them, we may, in a negative way, 'get approx¬ 
imately at their geological age of formation. For instance, if a 
series of veins penetrate the Archaean, they must have been 
formed during or after the Archaean. If penetrating the Car¬ 
boniferous, as at Leadville, after the Carboniferous, and so forth. 

But, though we can state with certainty that certain veins 
and ore deposits were formed after such and such an age, it is 
difficult to limit them to the exact interval of time in which they 
were formed. If, for example, we found a series of veins pene¬ 
trating to the top of the Archaean granite and ending abruptly 
at the Silurian, we might suppose that they were formed after 
the Archaean and before the deposition of the Silurian, but the 
evidence would be only negative, as there is nothing to show but 
that they may have been formed quite recently, and that the 
fissures only extended to the top of the Archaean without having 


GEOLOGY OF COLORADO ORE DEPOSITS. Ixxxiii 


force enough to break through into the Silurian. For instance, 
at Steamboat Springs, Nevada, veins are to-day being formed by 
hot waters coming up through a fissure in the granite. 

The ore deposits characteristic of Leadville are found in 
neighboring districts to extend as high as the base of the 
Triassic, or beginning of the Mesozoic era; therefore they must 
have been -formed as late as the opening of the Mesozoic. We 
have proofs, however, that they were formed before the great 
upheaval movement at the close of the Cretaceous, bee, the 
deposits are folded and faulted by that movement. 

Vein and ore deposits have been forming from a very early 
age up to the present time, and are still forming, as shown by 
the case of the Steamboat Springs we have alluded to. 

These springs are situated on a line of fissures extending in 
the direction of the celebrated Comstock mine fissure. The 
springs emit steam from a series of parallel fissures, which are 
lined with successive coats of silica, assuming the banded struct¬ 
ure observed in the veinstone of ordinary fissure veins. The 
fissure at one point has been opened beneath by a tunnel, and 
found to contain a quartz vein containing copper and iron pyrites, 
together with cinnabar and gold. Here we seem to have all the 
conditions of a fissure vein forming before our eyes. 

The great Comstock mine fissure appears to have had much 
the same origin, and the extraordinary heat that is met with at a 
depth of 3,000 feet is probably the approach to the retreating 
and dying geyser. It is possible that some of the fissure veins 
in the San Juan region may have been formed by the action of 
now extinct hot springs and geysers, as the whole region shows 
evidence of great volcanic activity in past time. 

ORIGIN OF ORE DEPOSITS. 

From the numerous theories that have been put forward to 
account for the origin of veins and ore deposits, we select such as 
seem to us most in accordance with the facts of this difficult 
problem. 

ERUPTIVE LODES. 

Metalliferous matter is sometimes disseminated through igne¬ 
ous eruptive rocks, as, for example, through the gold-bearing 
dykes of porphyry at Breckenridge. 


Ixxxiv GEOLOGY OF COLORADO ORE DEPOSITS. 

Metals occur in such rocks in minute particles, or as quanti¬ 
ties of various compounds of the heavy metal. In some cases 
these have been chemically dissolved out, and the metals again 
thrown down in such a way as to form valuable metalliferous 
deposits. In Australia, in South Park (Colorado), near Prescott 
(Arizona), and in various other places, such impregnated dykes 
have become decomposed. The originally disseminated gold has 
been deposited with quartz and other minerals, in the joints and 
fissures of the rock. In a certain sense these may be called 
eruptive lodes, and owe their origin indirectly to eruptive igneous 
agencies. 


SUBLIMATION THEORY. 

Vein fissures are supposed by this theory to be filled by the 
volatilization of metalliferous minerals derived from the ignited 
interior of the globe or from intense heat. This accounts for the 
unequal distribution of the ores in lodes by currents of dissimilar 
gases or • vapors circulating through fissures in the veinstone. 
These dissimilar vapors meeting, combine and precipitate various 
metals. Magnetite and specular iron are thus produced by 
decomposition of chlorides of iron, by watery vapors in fissures of 
modern volcanic rocks. Magnetite and galena have also so been 
formed by sublimation in the flues of smelting furnaces, and also 
tin oxide, zinc-blende, pyrites and other vein minerals. 

This theory, though it may contain some truth, does not 
account for earthy minerals forming veinstone in veins, nor vari¬ 
ous other phenomena connected with veins. 

Cinnabar, iron pyrites and gold are now, as we have said, 
being deposited from the waters of the geysers in Nevada. 
Quartz with its large crystals appears to be deposited by water 
only. 

LATERAL-SECRETION THEORY. 

This is one of the most important theories of modern days,, 
and is principally due to the labors of Prof. Sandberger. “ Water 
percolating through the country rock has by aid of carbonic acid 
and other solvents dissolved out of it all the materials now form¬ 
ing the constituents of veins.” 


GEOLOGY OF COL OF ABO OFF DEPOSITS. 


Ixxxv 


Some lodes in this connection show a marked difference as 
they pass from one variety of “country rock” to another. Sand- 
berger’s examination has proved this to be due to metals being 
derived from those rocks, and further that the metals were derived 
more particularly from certain constituent minerals of those 
rocks, such as mica, hornblende, augite and feldspar, the common 
•constituents of such rocks as granite, the porphyries, and other 
eruptive crystalline rocks. 

In each of these common minerals the elements of the metals 
occurring in metalliferous veins were detected and proof found 
that the heavy metals exist in the silicates of crystalline rocks of 
every geological age. 

In olivine, (a common crystal of basalt,) iron, nickel, cobalt 
and copper were found. 

In augite another constituent crystal of the same and other 
igneous rocks, lead, tin and zinc were found. 

Hornblende, common in granites and eruptive rocks, contains 
copper, arsenic, cobalt, lead, nickel, antimony, tin, zinc and 
'bismuth. 

White micas yield tin, arsenic, copper and bismuth. 

Black micas are not so productive, but yield similar metals. 

The feldspars, especially certain varieties of them, yield baryta 
and lime for veinstone, while their liberated silica yields the 
quartz. 

Thus, in these little minerals so common in nearly all rocks, 
especially those associated with our ore deposits directly or indi¬ 
rectly, we have all the elements necessary for the metals and the 
veinstone of veins and ore deposits, near at hand in the adjacent 
rock, without having recourse to the deep ignited regions of the 
earth, as required by the “igneous” and “sublimation” theories of 
the origin of veins. 

“Organic matter is sometimes present in veins in the form of 
graphite and anthracite, it also forms the coloring matter of smoky 
quartz, fluorspar, etc. This occurs when the amount of organic 
material originally present has been more than sufficient to trans¬ 
form the metallic sulphates into sulphides.” 

In the depths and near the surface of the Bassick mine at 
Silver Cliff, Colorado, a substance resembling charcoal is found. 


Ixxxvi GEOLOGY OF COLORADO ORE DEPOSITS . 


As lodes occur both in crystalline and semi-crystalline rocks, 
and also in rocks derived from these, such as sandstones, ores 
may be derived from the incompletely decomposed remains of 
metalliferous silicates such as mica, hornblende, etc., from the 
original crystalline rocks such as granite, or from solution pro¬ 
ducts from older veins, or from traces of metals which are found 
in sea water. 

Copper and gold have been detected in the waters of the 
Mediterranean. 

In gypseous deposits of ancient seas copper is sometimes 
present. 

In the red Trias sandstones of South Park, which were doubt¬ 
less deposited in inland seas or estuarine formations, copper stains 
are common. Copper is similarly found in modern estuarine 
muds. Stratified rocks such as sandstones, that have been formed 
by the sea, consist of the debris of older crystalline rocks such as 
granite, which contain, as we have said, both the elements of the 
heavy metals in their constituent minerals and also in veins in 
their mass, hence is not surprising that we find copper stains in 
the Trias conglomerate, and at Morrison we have even found 
large crystals of galena in the boulders composing the conglom¬ 
erate. Copper and zinc again have been detected in clay slates 
of marine origin. 

Some strata possess a composition enabling them to decom¬ 
pose metalliferous solutions derived from other sources more 
readily than others, and re-deposit their contents in the form of 
ores. Limestone, for example, not in itself a metalliferous rock, 
seems a favorite receptacle of ores or ore solutions from other 
richer or ore-generating rocks. 

ASCENSION THEORY. 

Lodes were formed in part only of minerals dissolved out of 
the adjacent country rock. The chief portion of the material 
was derived from greater depth by solvents circulating through 
the fissures, sublimation assisting either with or without steam. 
The increased heat and pressure due to greater depth enables the 
solution of different vein-forming substances and minerals to be 
deposited in all parts of the fissure of which the constituents do 
not exist in the rocks or the immediate vicinity. 


GEOLOGY OF COL OF A BO OFF DEPOSITS. Ixxxvii 


These solutions will, under the pressure, penetrate deeply 
into the surrounding rocks, and impregnate them with metal¬ 
liferous minerals, also softening and decomposing the rocks to a 
considerable distance from the lode, or they may cilicify the 
cheeks of the fissure. 

“Waters of solvent powers, increased by high temperature 
and pressure, percolating through rocks containing heavy metals 
will gradually remove them by lixivation, together with other 
mineral substances. These will again be deposited upon the 
sides of the fissure in proportion as the solvent powers of the 
mixture become lessened by diminishing temperature and 
pressure.” 

Minerals diffused through rocks near the surface may be 
removed by solutions which, penetrating into vein fissures, have 
mingled with the waters circulating through them. Deposits 
akin to those of true veins are at present being formed by the 
action of hot mineral springs. In the Steamboat Springs of 
Nevada such mineral veins are in process of formation. These 
springs issue from extensive fissures, which are being filled with 
silicous veinstone carrying oxide of iron and manganese, sul¬ 
phides of iron and copper and metallic gold. 

They also exhibit the banded structure so frequently observed 
in veins. A tunnel has been driven intersecting, at a great depth, 
one of the fissures formed by these springs, and a banded vein 
was found of quartz-carrying cinnabar. Sulphur is also found in 
these deposits and occurs in the old auriferous reefs of Australia 
and in some of the mines of Redcliff. Sulphuretted hydrogen 
may account for the formation of certain metallic sulphides in 
veins. 

ORIGIN OF LEADVILLE ORE DEPOSITS. 

The Leadville ore deposits have been the most thoroughly 
investigated by the U. S. Geological Survey under Mr. S. F. 
Emmons, of any of the mineral deposits of Colorado, and as the 
results of the investigation have a bearing upon many similar 
occurrences of minerals in the Rocky Mountains, we give an 
epitome of his views. 

“ The ores are deposited for the most part in the ‘ blue lime¬ 
stone’ of the Lower Carboniferous, As the ores were deposited 


Ixxxviii GEOLOGY OF COLORADO ORE DEPOSITS. 


by water solutions, the soluble limestone beds would be more 
easily acted upon by solutions than the sandstones and shales com¬ 
posing the other rocks of the neighborhood which are less 
susceptible to percolating water. The Paleozoic formations in 
America are the principal repositories for lead and silver ores, not 
by reason of their geological age so much as by their containing 
such a quantity of soluble limestones, and being physically as well 
as chemically favorable for the reception of mineral solutions. 

The physical, structural conditions of Leadville are particularly 
favorable to the concentration of percolating waters in the blue 
limestone. Great intrusive sheets of porphyry follow the lime¬ 
stone persistently, principally on its upper surface. This porphyry 
is very porous, and full of cracks and joints, affording ready 
channels for water from above, and also channels for ascending 
water from below along the walls of the fissures, through which 
it is erupted. Such waters passing through a medium of different 
composition would be ready for a chemical interchange with the 
limestone.” 


COMPOSITION OF ORES. 

The ores were deposited originally as sulphides. This is 
shown by the fact that the oxidized ores near the surface pass 
down with depth into sulphides. In Ten-Mile district these oxi¬ 
dized ores are seen to result from the alteration of a mixture of 
galena, pyrite, and zincblende. There is very little gold in the 
average Leadville ores; what little there is comes from the Flor¬ 
ence mine (native gold), and from others where it is associated 
with pyrites. It is usually associated with porphyry rocks, and 
a porphyry commonly called pyritiferous porphyry shows gold 
to exist diffused through the pyrites disseminated through its 
mass. 

Silver occurs as chloride, a secondary condition, its original 
condition probably being sulphide. 

Lead occurs as carbonate and sulphate, and deep in the mines 
as sulphide. Specimens are common of galena nodules sur¬ 
rounded by a thin coat of sulphate, and that again by a coat of 
carbonate, showing the order of transition from sulphide to sul¬ 
phate and thence to carbonate. 


GEOLOGY OF COLORADO ORE DEPOSITS. Ixxxix 


In the Iron mine native sulphur occurs as an alteration 
product of galena. 

Iron and manganese constitute rather a gangue material than 
an ore. They are hydrated oxides and protoxides. The iron 
was originally deposited as sulphide or pyrites, but has been 
wholly transformed by oxidation. 

Zinc is not common, but occurs as calamine (zinc silicate) in 
needle like hairs and white crystals in cavities in the mines. Its 
original form was zincblende (zinc sulphide), as shown in the 
Ten-Mile district. 

The earthy minerals, alumina, lime, silica and magnesia, are 
in fair proportions, as might be expected from ores which are a 
replacement of limestone in close connection with porphyry. The 
alkaline element among the ores might also be traced to the 
influence of the latter rock. 

The agents of alteration were surface waters, which contain 
everywhere carbonic acid, oxygen, organic matter, chloride of 
sodium (common salt), and phosphoric acid. The rocks through 
which these waters passed, such as porphyries and limestones, 
were found to contain phosphoric acid and chlorine, while organic 
matter exists in the blue limestones, and in the overlying shales 
and sandstones are many carbonaceous beds and even beds of 
coal. Water passing through these rocks would take up all 
these elements and be ready for chemical reactions. 

Galena (lead sulphide) is much richer in silver than its alter¬ 
ation product, carbonate of lead, or cerussite. On Carbonate 
Hill the carbonate averages 40 oz. silver, the galena is 145 oz. to 
the ton. But galena is harder of treatment. 

Silver is found at times disseminated through vein matter and 
country rock, without the presence of lead, proving that during 
alteration silver was remoyed farther from its original condition 
and more widely disseminated than lead. 

Outcrop deposits have proved in many cases richer than those 
at depth. The deposits near the surface have been the refined, 
concentrated remains of larger bodies gradually removed by 
erosion as the alteration by surface waters went on. The baser 
and more soluble metals have thus been removed in solutions, 
leaving behind the more valuable and perhaps less soluble metals 
in new and richer secondary combinations. 


xc 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Kaolin or Chinese tale , which occurs both along the line of 
contact and between the porphyry and limestone and also in the 
heart of the vein material, is a decomposition product from por¬ 
phyry. It consists principally of hydrated silicate of alumina, 
doubtless derived from the feldspars of the porphyry, perhaps at 
the time when acted upon by sulphurous waters which brought 
in the original ore deposits. 

Calcite occurs incrusting recent crevices and lining recent 
cavities, but in small amount. 

Barite or heavy spar is common, generally associated with 
chloride of silver and with manganese, and is locally recognized 
as a sign of rich ore. 


ORE DEPOSITED AS SULPHIDES. 

We have already stated that depth in the mines away from 
the surface waters proves this to have been the original character 
of the deposits. 

Under what reaction could this occur? 

Sulphides of the heavy metals may be precipitated from vari¬ 
ous solutions. 

1st. Where they exist as sulphides, by sulphides of the alkalies 
and alkaline earths. 

2nd. Where they exist as carbonates and sulphates coming 
in contact with solutions containing alkalies and sulphuretted 
hydrogen. 

3rd. Where they exist as sulphates, which in contact with 
organic matter are reduced to sulphides. 

Metallic sulphides are soluble in water containing alkalies or 
sulphuretted hydrogen or silica, and in waters containing alkaline 
carbonates. 

Solfataric or hot waters, arising from the heated depths, con¬ 
tain sulphuretted hydrogen, alkaline sulphides, and carbonates. 

If the metals of these deposits came up from the heated 
depths or were derived from pyrites and galena in neighboring 
rocks, then the iron and lead were brought in as sulphides. This 
would seem to involve that the carbonates and sulphates of the 
limestone should have been dissolved out and carried away before 
the sulphides were deposited, and this would involve the popular 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xci 


pre-existing cavity theory (which Mr. Emmons believes to be 
incorrect). Probably, however, the dissolving out of the former 
so immediately preceded the deposition of the latter, that the pro¬ 
cess was an interchange of substance for substance, or the com¬ 
mencement of a change from sulphide to sulphate may have 
taken place in presence of the carbonate, and the sulphate been 
immediately reduced to sulphide again by organic matter, and 
there is evidence that locally the dolomite limestone has been 
directly replaced by sulphides, by zincblende, pyrite and galena 
in pseudomorphs after calcspar and dolomite. 

In contact with dolomite containing organic matter the sul¬ 
phates would be reduced to sulphide with the formation of 
carbonic acid. The waters thus charged with carbonic acid would 
dissolve and remove the carbonates of lime and magnesia, which 
would be replaced by metallic sulphides. 

Any excess of sulphuric acid would form soluble sulphates 
of lime and magnesia, which would also be carried away. If 
these sulphates were reduced to sulphides they would render the 
waters more capable of dissolving out the dolomite. The metals 
might have been taken up in the form of sulphates by waters 
percolating through rocks, where they might have been brought 
into this combination by the oxidation of sulphides, or by 
decomposition of silicates, or in this transition the sulphates may 
have been reduced to sulphides by contact with organic matter 
before reaching the locality of deposit. 

Sulphide of barium would be precipitated as sulphate of 
baryta in contact with limestones. 

Silica brought in by waters containing alkaline carbonates, is 
soluble, and might form silicates of the alkalies, carbonic acid 
waters carrying away earthy carbonates. 

Later, the combined alkalies were replaced by oxide of iron, 
and in part dissolved out, leaving free silica. 

MODE OF FORMATION. 

The Leadville ores, like most others, were deposited from 
water solutions by a metasomatic interchange, i. e ., substance 
exchanged for substance with the limestone, and lastly or orig¬ 
inally, as sulphides. 


xcii GEOLOGY OF COLORADO ORE DEPOSITS. 

Mineral matter is carried from one place to another within 
the earth’s crust by heat and water, or these combined. Met- 
asomatic interchange of metal for limestone and the removal of 
dolomite could only have been produced by water. The ores 
were not deposited in pre-existing cavities, but are a replacement 
of the country rock, i. e., dolomitic limestone. 

The ores grade off gradually into the material of the lime¬ 
stone, without a definite limit, as would have been the case if the 
limestone had been previously caverned. The only limiting out¬ 
line to the ore bodies is that formed by the contact porphyry. 

Fragments of unaltered limestone are found entirely enclosed 
within the ore bodies, and ore bodies often occupy the entire 
space for long distances between two horizontal sheets of 
porphyry, which space further on is occupied by the limestone. 
(This is well seen in the Colonel Sellers mine.) Examination of 
ores and veinstone show lime and magnesia not in the crystalline 
condition they would have, had they been brought into a pre¬ 
existing cavity and deposited, but in the same granular condition 
in which they exist in the country rock. 

“The deposits in rocks other than limestone consist of 
metallic minerals and of altered portions of the country rock, in 
which the structure of the latter can sometimes be still traced, and 
are not the regular layers of matter foreign to the country rock, 
which results from the filling of a pre-existing fissure or cavity 
by materials brought in from a distance and deposited along the 
walls. 

In the Ten Mile district the arrangenient of the particles of 
the original rock is frequently seen to be preserved in the 
metallic minerals which maintain a certain parallelism with the 
original bedding planes in the lines defined by minute changes in 
these minerals. 

The common character of caves which have been dissolved out 
of limestone, is that their walls are coated with a layer of clay 
which has been left undissolved by the percolating waters, and 
these walls have a peculiar surface of little cup-shaped irregular¬ 
ities from which also stalactites frequently hang. There is also 
an accumulation at the bottom of the cave of fragments of lime¬ 
stone fallen from the sides of the roof. None of these character¬ 
istics are found associated with the ore re-placements. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xcm 


Also, when mineral matter is deposited in “ pre-existing cav¬ 
ities ” it takes the form of regular layers parallel with the walls of 
the cavity, as is beautifully shown in geodes lined with a succes¬ 
sion of zeolites or with layers of chalcedony, opal, and quartz. 

No such successive arrangement in layers is found in the 
Leadville ore bodies. 

Again, could such large, open cavities have existed for long 
distances without support, between the layers of porphyry? 
Why did not these porphyry sheets close together ? And 
further, how could such extensive cavities have been formed and 
kept open under a pressure of 10,000 feet of rock, which 
the geology of the region shows to have existed above the 
deposits, at the time they were being formed? Such cavities as 
we do find in the region are all of very recent origin, cutting 
through both limestone and ore bodies, and have been hollowed 
out by surface waters more recent even than those which pro¬ 
duced the secondary alterations in the ore bodies. 

ORIGIN OF METALLIC MINERALS. 

Ore deposits have been deposited from solutions through the 
agency of water, with or without the assistance of heat. 

Within the rocks forming the crust of the earth there is a 
constant circulation of waters, carrying more or less mineral 
matter in solution, and no rock is absolutely impermeable. 

There are upward and downward currents. The latter are 
surface waters sinking by gravity. The former are the same 
waters rising under the influence of the heat of the earth. The 
direction which such waters take will depend upon the struc¬ 
ture of the rock mass through which they pass, whether upward, 
downward or laterally. 

Waters filling capillary passages and minute fissures will seek 
larger channels in joint, fault, and stratification planes. Water 
carrying mineral matter in solution along such channels will 
deposit it where the rock favors chemical precipitation or inter¬ 
change, and this will take place most where there is some interrup¬ 
tion in the flow, as rapid waters deposit less readily than those 
whose movement is slow. 


XCIV 


GEOLOGY OF COLORADO ORE DEPOSITS. 


SOURCE OF METALS. 

“ The ultimate source is a matter of speculation, like the neb¬ 
ular hypothesis, by which the earth is supposed to have arrived 
at its present condition as the result from the gradual cooling of 
an incandescent mass, and as the specific gravity of the crust is 
much less than that of the whole mass of the earth, it has been 
inferred that the heavy metals must be in much larger proportion 
in the interior of the earth than in the rocky crust,” (though this 
greater specific gravity might be also accounted for by the rocks 
of the interior being much more tightly packed by pressure than 
those near the surface). 

“Volcanic emanations and hot springs contain metallic min¬ 
erals, so also do the waters of the ocean. But we know not from 
what depth the former came, nor from what source the latter 
derived them. As circulating waters take up and throw down 
their metallic contents under varying conditions, the same material 
may have been deposited more than once and in more than one 
form, since it reached the rocky crust. The ores do not seem to 
have ascended from below, for the geology of Leadville shows 
that the ores were deposited beneath a thickness of not less than 
10,030 feet of strata and an unknown depth of sea-water. If they 
had been deposited from hot ascending solutions as the result of 
a relief of pressure, which would favor precipitation, the deposit 
would be found in the upper part of this mass of rock rather than 
at its base. 

The sedimentary beds at the time of deposit were horizontal 
and undisturbed. There are no channels discovered through 

o 

which the ore could have ascended, the eruptive rocks being hori¬ 
zontal and parallel with the stratification, hence the ore deposits 
were not formed by ascending waters. 

The principal water channel at the time of deposition was the 
contact of the upper layers of the blue limestone with an over- 
lying sheet of porphyry, and from this surface they penetrated 
downward into the mass of limestone. The currents were de¬ 
scending by gravity rather than ascending by heat. 

Percolating waters circulate freely through eruptive rocks 
owing to their porous character, and their tendency to jointage 
and fracture, the results of cooling and weathering. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


xcv 


In sedimentary rocks the bedding planes are the natural 
courses for water. 

Water would thus descend through the vertical joints of the 
overlying porphyry, and would turn off horizontally on or between 
the bedding planes of the underlying limestone into which they 
would eat their way gradually downwards, through jointage cracks 
and other weak points. 

According to the “lateral secretion” theory, which seems 
nearest to the truth, all the substances contained in the lodes have 
been derived from the adjoining rocks. Sandberger and others 
have satisfactorily shown this to be the case, and that all metals 
found in veins can also be found in minute quantities in the con¬ 
stituent minerals such as mica, hornblende, augite, and other 
silicates of the adjacent or country rock. And, rather singularly, 
he considers the metals are derived more from these silicates than 
from the pyrite so commonly diffused through eruptive rocks, 
considering, moreover, that the latter are not original constituents 
of these rocks. 

The results of a minute analysis of the rocks adjacent to the 
ore bodies at Leadville, viz., porphyry and limestone, selecting 
fragments the most remote from ore currents and the purest ob¬ 
tainable, show baryta and lead to be contained in the feldspars of 
the porpyhy. Gold and silver also exist in minute quantities in 
the elements of the porphyry, especially of that variety called 
pyritiferous porphry, and those porphyries and diorites which con¬ 
tained the greatest amount of basic elements as a rule yielded 
the greater proportion of the heavy metals. The percentage 
of these metals is very small apparently, but when we consider 
the enormous amount of the porphyries, (some even 1,000 feet 
thick,) actually present, not to mention those that have been re¬ 
moved by erosion, we see that this small percentage, if concen¬ 
trated, would adequately account for all the metal found in the 
Leadville region, with a good deal yet to spare, minutely dissem¬ 
inated or chemically combined with certain earthy minerals con¬ 
stituting the elements of the great bodies of porphyry. 

On the other hand the dolomitic limestone and other sedi¬ 
mentary rocks were found in themselves to show no original heavy 
metal elements. 


XCVl 


GEOLOGY OF COLORADO ORE DEPOSITS. 


The metals then, came from the porphyry and were deposited 
only in the limestone. Some of them, too, may have ascended 
through the dykes that were feeders to the great horizontal 
porphyry sheets.” 


Part III. 


PRINCIPAL COLORADO MINING DISTRICTS. 


BOULDER COUNTY. 

The mines are situated on the Eastern slope of the Colorado 
range. The mining district is about thirteen miles long by four 
to ten wide (not including Caribou Hill). 

GEOLOGY. 

Along the foothills adjoining the plains is a series of “hog- 
backed ” ridges formed by the upturned floor of the prairie, con¬ 
sisting ofTriassic, Jurassic and Cretaceous‘strata, resting on the 
Archaean granite core of the range. 

These upturned sedimentary beds of sandstone, limestones, 
clays and shales form a fringing belt, varying in width and dip, 
along the entire extent of the Eastern foothills. 

South of Boulder their dip is almost vertical, forming near 
South Boulder Canon a magnificent peak 3,000 feet above the 
plains. 

The Upper Cretaceous or Laramie group contains valuable 
coal beds, whose outcrops, owing to erosion at Boulder, are some 
miles out on the plains. 

These hogbacks also supply excellent building stone, flag¬ 
stones, fire-clay and lime. 

They are barren, however, ot precious minerals, both here 
and generally along the Eastern foothills. 

The Archaean granite rocks immediately adjoining the plains 
have also, as a rule, been found to contain but few valuable 
minerals. 




xcviii GEOLOGY OF COLORADO ORE DEPOSITS. 

A few copper stains and some local stains and deposits of 
copper are nearly all that is found. It is not until the range has 
been penetrated for a distance of several miles that productive 
deposits appear. 

In Boulder County we have the nearest mines to the plains, 
they being not more than two miles distant. 

The Archaean rocks consist principally of a granite-gneiss, 
showing indistinct signs of primitive stratification. This is inter¬ 
sected by veins of pegmatite, or very coarse, crystalline, and 
sparry granite, varying in width from a few inches to 40 or 50 
feet. Their composition is the same as granite, consisting of 
white feldspar and quartz, with very little mica; in other words, 
granite in a coarser and more crystalline, sparry condition than 
the adjacent country rock and with less of its mica. 

Two of these veins, the Maxwell and the Hoosier, are strong 
and well defined, traversing the district for several miles. The 
Maxwell runs East of North, crosses the road two miles from 
Boulder on the way to Sunshine, and is easily visible from its 
reddish, white and rusty color. It carries pyrites and tellurides. 
The Hoosier vein, or rather gangue, forms the western limit of 
the telluride belt, is 30-feet wide, and runs through Gold Hill in 
a direction East of North. It carries silver ore and gray copper. 

The Telluride belt underlies the Magnolia, Sugar Loaf, Gold 

Hill and Central districts. Eruptive rocks are scarce in this belt, 

* 

but “pegmatite” veins abound. 

West of this region enormous masses of eruptive rock occur, 
and tellurides are not found. 

In the Caribou district are rich silver ores, carrying 300 to 
1,500 ounces silver to the ton. In the Ward district veins carry 
free gold, with iron and copper pyrites, which have a general 
direction East and West, while the others are more nearly North 
and South. Of eruptive rocks, that which forms the Sugar Loaf, 
a conical hill between Four-Mile and Boulder Creeks, is a fine¬ 
grained porphyritic rock, of a grayish color, showing small, 
white feldspars, black mica and hornblende, and crystals of titanic 
iron, with a little augite. The crystalline ground mass in which 
these crystals are set consists principally of feldspar, with a little 
quartz. A similar rock is on Four-Mile Creek, showing large ' 


GEOLOGY OF COLORADO ORE DEPOSITS. 


XCIX 


feldspar crystals. This rock is a massive eruption of con¬ 
siderable extent. A dense black rock not unlike basalt occurs 
east of the Sugar Loaf, in a dyke, and is called diabase. 

At Jimtown a quartz-diorite dyke occurs, of light color, con¬ 
taining much hornblende and titanic iron, running nearly through 
the street of the village. The cliffs at Jimtown, over 500 
feet high, are quartz porphyry, of a white color. It consists 
mainly of large crystals of quartz and feldspar, set in a fine¬ 
grained crystalline ground mass or paste. 

MINES. 

Boulder mines are celebrated for the occurrence of Telluride 
minerals, some of the richest and rarest ores occurring in nature. 
The Telluride belt occupies the eastern part of the district, 
and extends to within a short distance of the sedimentary “hog¬ 
backs.” It comprises the Magnolia, Sugar Loaf, Central and 
Sunshine districts. West of this belt no tellurides occur. 

In Caribou district the ores are rich argentiferous galena, with 
many varieties of other rich ore, stephanite, proustite, and others. 
In the Ward district pyrites abound, and where it is decomposed 
free gold is found. The pyrites, though gold-bearing, is difficult 
of reduction. 

The Boulder district contains very rich ores, yet development 
has been irregular and production uncertain, due partially to the 
irregular manner in which the ores occur. Of late, mining has 
been resumed at Caribou, with prospect of steady output. 

The veins, that is the “pegmatite” gangues, are called true 
fissures, and stand at a high angle and are often very wide, 
but the rich ore is concentrated in thin streaks and not very con¬ 
tinuous bodies. Of the character of the fissure veins Mr. Emmons 
says : “If the term ‘true fissure’ means a vein which occupies what 
was once a deep-seated, wide-gaping fissure, filled in by vein 
matter and ore, coming from unknown depths, and distinct and 
foreign to the material of the adjacent country rock, there are no 
such true fissure veins in this district,” and we might go further 
and add nor in Colorado generally. 

The gangue or vein material is simply an alteration of the 
adjacent granite or gneissic country rock, as testified by its com- 


c 


GEOLOGY OF COLORADO ORE DEPOSITS. 


position, which is quartz, feldspar and some mica. This is 
impregnated with rich mineral, whose source we may venture to 
say is not far to find, microscopically or chemically diffused in an 
adjacent country rock of porphyry, and concentrated in the sparry 
material. 

This impregnation has taken place either along the contact of 
an eruptive porphyry rock with the country rock granite, or else 
in a pre-existing vein of pegmatite, or along some fault or joint¬ 
ing plane in the country rock itself, which has been favorable to 
the concentration and precipitation of metallic minerals from their 
solutions. This account will fit many of the so-called “ true 
fissure veins ” in Colorado. The direction of the vein is generally 
between North-East and North-West, in the Ward district East 
and West. Their dips are mostly very steep or vertical. 

TELLURIDE ORES. 

The quartz or pegmatite gangue impregnated with telluride 
ore has generally a pale, bluish-gray and rather greasy appear¬ 
ance, streaked here and there with a dull, blackened, greasy stain 
upon which sometimes the true telluride minerals can be seen, such 
as sylvanite, which occurs in long, thin crystals of a bright, tin-like 
appearance. It is sometimes called graphic tellurium, because 
the crystals crossing one another assume the form of Hebrew 
writing characters. It is a telluride of gold and silver. 

There are many varieties of telluride ores, some rich in silver, 
and others in gold, and some with both combined. When a 
piece of the gangue containing tellurium is roasted, the gold will 
come out in good-sized globules on the surface. This used in 
early days to be a much-coveted specimen for those who wanted 
to possess a piece of Colorado gold in the rock itself. 

Hessite, petzite, Coloradoite, and native tellurium are among 
the varieties of tellurides. 

Central District. The “Golden Age” mine, near Jimtown, is 
at the contact of porphyry and granite. The vein is 40 feet wide- 
The ore comes from a streak of white quartz, one to two feet 
thick, sometimes very rich in free gold. Pyrites also occur. 
Rich concentrations of ore are found at intervals, some ore as 
high as $30 per pound; average ore mills $20 per ton. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


ci 


Gold Hill district is in the telluride belt traversed by the 
Hoosier gangue. Several of the veins cross the Hoosier gangue, 
and are richer in its vicinity. The Red Cloud’s vein is 3 ]/ 2 feet 
wide. The ore was telluride at the surface, passing into aurifer¬ 
ous pyrites in depth. 

Sunshine, also in the telluride belt. Its ores are lower grade. 
Free gold and tellurides occurred on the surface, passing into 
pyrites with depth. The American, Grand View and others are 
among the principal mines. 

Sugar Loaf, also in the telluride belt, is an enrichment of the 
Hoosier gangue, the gangue being pegmatite. 

Magnolia district. In the Keystone mine is a narrow deposit, 
6 to 7 inches wide, yielding Coloradoite (telluride). 

Ward district is outside of the belt, and carries copper and 
iron pyrites bearing gold, some of which mills $ 60 per ton on an 
average. 

Caribou. The Caribou mine has yielded a great deal of silver. 
Its ores are a mixture of galena, chalcopyrite and zincblende, 
occurring in gneiss near a dyke of eruptive diabase. The Car¬ 
ibou mine has produced two millions of dollars. 

The “ No-Name ” is said to cross and fault the Caribou. The 
ores are silver-bearing, but also carry gold. Ores are silver 
glance, stephanite, gray copper, argentiferous galena, copper 
pyrites. Native silver is common, also some ruby silver. Copper 
pyrites carry more gold than silver. 

Prof, van Diest considers that the granite rocks near Boulder 
are thrown into a series of parallel folds, “ first a great fold fol¬ 
lowing the continental divide, prominent near Gold Hill ; another 
near North St. Vrain, and a third between Middle and South 
Boulder. Also, two prominent side folds cut these main folds 
diagonally, one running from Ballarat to Jimtown, the other from 
Sugar Loaf to Gold Hill. The telluride veins run along the 
slopes of these folds.” 

He appears to associate the veins with cracks and fissures 
coinciding with this folding. “ Some of the main fissures being 
filled at once by porphyry dykes, the others more gradually by 
vein material.” “ The veins,” he says, “ occur along, on, and near 
these dykes, along lines at the junction of the more massive 


Cll 


GEOLOGY OF COLORADO ORE DEPOSITS. 


granite with the stratified gneiss, along and between stratification 
planes of schist, and along the joint planes of granite.” 

He attributes the veins to percolating alkaline waters dissolv¬ 
ing metalliferous material and veinstone from the surrounding 
rocks. Alkaline springs, he observes, still exist in the neighbor¬ 
hood, as they do in the mining district of Idaho Springs. “ The 
veins occur where the foldings are abrupt, and the direction of 
the veins is parallel to the strike of the stratification. As a rule, 
the veins in Boulder County are not of great extent; a single 
vein can rarely be traced on the surface or beneath it for more 
than 600 feet. Before that distance is reached the vein spurs off 
into another vein, follows it for a while, and spurs off again into 
another. 

“Where veins cross at a small angle or where a spur branches 
off from the main vein, accumulation and enrichment of ore takes 
place. There are two courses of veins, one East and West, the 
other North-East by South-West. The former system appears 
to be the oldest, as the latter faults it. 

“ The ore occurs in chimneys or pockets, with a good deal of 
nearly barren ground between. Small veins run parallel with 
each other for some distance, the interval filled with granite or 
pegmatite. Sometimes a vein pinches out entirely. The ore 
streak is from 1 to 20 inches wide, containing more horn- 
quartz than the country rock. Some of the veins interlace like 
arteries in a body. Minute particles of pyrites (marcasite) often 
produce a dark stain in the telluride quartz. By moistening the 
stone the telluride minerals and pyrites appear distinctly.” 

GILPIN COUNTY. 

The geology of the mines and veins of Gilpin County, which 
congregate around the vicinity of Central City and Black Hawk, 
resembles that of Boulder. The region consists of Archaean 
granite and granite-gneiss, penetrated by felsite and quartz por¬ 
phyry dykes. The veins are here also only alterations of the 
country rock along certain planes, but do not occupy a once 
wide, gaping fissure. In some mines th£ vein material is a por¬ 
phyry dyke. The vein of the Minnie mine is a felsite porphyry; 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cm 


of the Cyclops mine, a quartz porphyry. Dykes of porphyry 
occur near the lodes or in contact with them. 

The veins have been traced to a considerable depth, over 1,000 
feet, and in length over 3,000 feet. The direction of veins is 
between North and South and North-East and South-West. The 
dip is nearly vertical. 

The ores are a mixture of iron and copper pyrites, very little 
galena, and some zincblende. All carry more or less gold. 

There is also a silver district in the northern portion. The 
diameter of the gold district, which is quite distinct, is not more 
than 1 x / 2 miles. 

In the gold veins the richer ore occurs in streaks not over 
one foot wide, in a compact, fine-grained mass of pyrite, copper 
pyrites being richer than ordinary iron pyrites. The rest of the 
vein, often many feet wide, carries pyrite irregularly, dissemi¬ 
nated through decomposed country rock. The bulk of these 
ores are difficult to treat and are milled, the loss being 40 per 
cent, higher in the unoxidized than in those completely oxidized. 

The veins follow the cleavage planes of the country rock. 
These cut the stratificated planes at right angles, with a vertical 
dip. It is supposed that the porphyry dykes are older than the 
veins, as the cleavage intersects the porphyry equally with the 
other strata. 

The interval between these mining districts and the plains, 
usually 20 miles or more, is commonly barren of precious 
minerals. 

The Gregory, Bobtail, Burroughs and others are among the 
mines of note. 


CLEAR CREEK COUNTY. 

“The geology of this adjacent district is much the same as 
that of Gilpin county. The country rock is Archaean granite and 
gneiss, traversed by porphyry dykes. The fissure veins are also 
alterations of the country rock along a jointing or faulting plane 
and are frequently in direct connection with porphyry dykes which 
form either one or the other wall of the vein and sometimes consti¬ 
tute the vein material itself. In other cases the mineral vein is 
an impregnation of a pre-existing pegmatite vein in the gneiss.” 


CIV 


GEOLOGY OF COLORADO ORE DEPOSITS. 


“The ores are silver bearing and derived from argentiferous 
galena and gray copper. Where pyrites abound the ore yields 
both silver and gold. The rich ores are smelted. A large pro¬ 
portion is concentrating ore which impregnates the country rock 
at a greater or less distance from the main crevice, usually on the 
foot-wall side. 

The porphyry filling or gangue of the Colorado Central vein 
assays 0.063 oz - °f silver to the ton, and a trace of gold. 

Georgetown, Idaho Springs and Geneva Gulch are the centers 
of the principal districts. 

Geneva Gulch and Hall Valley though not in Clear Creek 
county belong to the same mineral belt.” 

Obsidian dykes occur in the Colorado Central vein parallel 
with the vein, which is a porphyry dyke, there is therefore a dyke 
of obsidian within an impregnated dyke of porphyry, and the 
richest mineral is close to that obsidian. 

The Centennial has one wall porphyry, the other not found 
and the mineral lies close to and impregnates the porphyry, fading 
out in the same rock. The porphyry assays a fraction of an ounce 
gold and silver in the Centennial, and three to four ounces gold 
in the Colorado Central. Feeders come in, and the best ore is 
between the feeders, but not in the feeders themselves. The ores 
are copper and iron pyrites, and, near the granite, zinc and lead. 

In the Colorado Central mine faulting seems still progressing. 
Mr. C. Gehrman tells me they are obliged to re-timber the mine 
every now and then in consequence of the foot-wall rising. Some 
of the Georgetown veins between walls are quite large, (from 50 
to 100 feet,) but the pay streak, though rich, is small in pro¬ 
portion. 

The Centennial vein is large and carries plenty of ore, but not 
of very high grade. There are three main porphyry dykes in the 
region with branches from them. The gold ore keeps near the 
porphyry and is an impregnation of it. 

SUMMIT COUNTY. 

“The high mountain portion of this county consists of Archaean 
granite rocks. But along the valley of the Blue river, fragmen¬ 
tary beds of the Silurian, Carboniferous, Triassic, Jurassic and 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cv 


Cretaceous periods occur which have escaped erosion, relics of a 
former connection of the seas which filled the South and Middle 
Parks. 

These rest on the Archaean of the Park Range, and are 
repeated on its West side, the park range having been lifted up by 
the great fault movement so well defined in the Mosquito range. 

Along the upper portion of Eagle river, Silurian and Carbon¬ 
iferous beds appear, dipping North and resting on the Archaean 
granite of the end of the Sawatch range, associated with these 
are a great many eruptive porphyry rocks, the latter throughout 
this district show a marked connection with the relative richness 
in size of the ore deposits which occur all the way from the 
Archaean to the Triassic. 

At the head of the Blue River the Silurian, Carboniferous and 
Triassic formations have been much traversed by eruptive sheets, 
whose heat has caused metamorphism of the sedimentary beds. 
The beds are also much faulted, and the principal developments 
center around Breckenridge. 

The “Helen” mine in French Gulch has ore as an impregnation 
of quartzite for 45 feet in width. The ore is free gold with some 
silver. The quartzite is rusty by the leaching out of the aurifer¬ 
ous pyrites it originally contained. In the McKay mine argen¬ 
tiferous galena and carbonates of lead occur near an overlying 
bed of porphyry in a sedimentary rock.” 

The Monte Cristo mine on Quandary Mt., has a deposit of 
low grade galena and zinc-blende, impregnating Silurian quartzite, 
its average is 15 oz. to the ton. 

Veins occur at several points in the Archaean granite of the 
Mosquito range, but so far unimportant. 

In Ten-Mile district the ores are mainly in the Upper Car¬ 
boniferous limestones and sandstones, a higher horizon than at 
Leadville. This is an area of wonderful eruptive activity abound¬ 
ing in intrusive sheets and dykes of porphyry. The ores are 
rather low grade and refractory, consisting principally of pyrites 
mixed with zinc blende. Most of the ore bodies occur in thin 
beds of limestone at their contact with a micaceous sandstone, 
more rarely at contact with a bed of porphyry or impregnating a 
dyke of porphyry. The last is best seen at the Pride-of-the-West 


CVl 


GEOLOGY OF COLORADO ORE DEPOSITS. 


on Jacques Mt. The Robinson is the principal mine, its ore 
is high grade argentiferous galena, associated with pyrites and 
zinc-blende, it occurs near the surface of a bed of gray limestone 
over-laid by white micaceous limestone, dipping N. 17 0 . The 
ore is a re-placement of the limestone. The upper layer, con¬ 
sisting of pyrites and white mica, is a replacement of the over- 
lying sandstone and is worthless. Below this the ore consists ot 
galena and pyrites, extending to irregular depths in the limestone 
and in the larger bodies occupying its whole thickness. The great¬ 
est width of the ore chute, 100 feet, has been traced 1,000 feet 
following the dip. A crack or fault plane in the roof follows the 
line of the ore body and probably furnished the channel through 
which the ore solutions reached the limestone, as pyrites extend 
all through the fissure. 

On Elk Mountain ore is found in a thin bed of limestone at a 
higher horizon still than the Robinson, but it is poor in silver and 
it even extends up into the Triassic red sandstones. The “Pride- 
of-the-West,” on Jacques Mountain, is a dyke of porphyry im¬ 
pregnated with baryta and ore. 

On Eagle River, near Redclift, argentiferious galena and 

carbonate of lead with iron oxide occur between limestone and 

* 

porphyry or between limestone and quartzite. The limestones 
are carboniferous.” 


PARK COUNTY. 

The basin plain of South Park is covered by sedimentary rocks 
of Triassic and Cretaceous age underlaid by Carboniferious and 
Silurian formations. These slope up to the crest of the Mosquito 
range on the West, but are apparently cut off abruptly against 
the Archaean granite on the East, probably by a fault. The coal 
beds of the upper Cretaceous occupy a portion of the center of 
the park around Como and stretch Southward to the Platte River. 

Near Hamilton are deposits of hematite iron. Salt springs 
occur in the Southern end of the park, issuing from Triassic red 
sandstones. 

In the North-East corner of the Park, in the granite rocks, 
are the Hall valley and Geneva districts, a continuation of the 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cvn 


Clear Creek silver belt system. In the “ Whale ” mine the gneiss 
is intersected by numerous veins of pegmatite. The lode runs 
North-East and South-West, and dips at 65°. It is a thin vein 
of baryta and quartz, with irregular bunches of galena and gray 
copper, the latter very rich in silver. The crevice is 5 to 10 feet, 
but the vein proper, or pay streak, is from an inch to three feet 
wide. The altered walls are impregnated with pyrite, galena and 
zincblende. The principal mineral developments are along the 
eastern slope of the Mosquito range, and are derived principally 
from Silurian and Lower Carboniferous rocks. The order and 
succession of the lower, older or Paleozoic rocks composing this 
range are here seen, together with their average thickness. 

First granite, forming the base and usually found at the 
bottom or on the cliffs of the deeper canons; upon this rests : 

FEET. 

J ^ Cambrian quartzite.. . 200 

~ j Silurian white limestone.200 

C/2 [ 

( / Lower Carboniferous blue limestone . . . 200 

C/3 I 

§ \ Middle Carboniferous, or Weber grits (sand- 

~ / stones and quartzite).2,000 

■g j Upper Carboniferous limestones, reddish 
° \ sandstones and conglomerates .... 1,000 

\ Total.3,600 

In some localities the total will reach 4,000 feet. These 
formations have been traversed by eruptive quartz porphyry, 
porphyrite dykes and intrusive sheets. The dykes occur princi¬ 
pally in the Archaean, but the intrusive sheets are many, spread 
out between the quartzites and limestones of the Silurian and 
Carboniferous. 

The connection between the eruptive masses and deposition 
of ore is very marked. “ The ore bodies are a concentration ot 
the metallic minerals originally disseminated through the mass 
of these eruptive porphyries, and now deposited along their plane 
of contact with the sedimentary beds, and extending more or less 
into the mass of the latter.” 

On Mts. Lincoln and Bross, in the principal mines, such as the 
Moose, Dolly Varden, Russia, and others, the ores are mainly 







cviii GEOLOGY OF COLORADO ORE DEPOSITS . 

argentiferous, yielding galena and its products of decomposition, 
viz., carbonate of lead (cerussite) and sulphate of lead (anglesite), 
with chloride of silver. Barite (heavy spar) is a common gangue 
or veinstone, especially in the richest parts of the mine. Iron 
pyrites, decomposed and passing into a hydrated oxide of iron, 
together with a black oxide of manganese, give to the ore its 
rusty and black look. 

The deposits occur in irregular bodies or pockets, often of 
great size, in the blue limestone, near its upper surface, but not 
always easy to find or follow. This limestone was originally 
covered by a sheet of quartz porphyry, which has been locally 
removed from the ore deposits, but exists in various parts of the 
Mt. Lincoln peak. The quartz porphyry is the variety called Mt. 
Lincoln quartz porphyry, and recognized by its large crystals of 
feldspar. The age of this porphyry is probably as late as the 
Cretaceous. As in the Gunnison, it is found breaking through 
rocks of that period. In the Dolly Varden mine the ore occurs 
in the limestone at contact with a vertical dyke of white quartz 
porphyry. In the Fanny Barret mine, on Loveland Hill, rich 
deposits of galena and anglesite occur in a vertical fissure or 
jointing plane traversing the limestone at right angles to its dip. 

In Buckskin Gulch the Phillips mine is an immense mass of 
gold bearing pyrites, deposited in beds of Cambrian quartzite near 
a dyke of quartz porphyry. 

The Criterion, on the cliffs of the gulch consists of large cav¬ 
ities in quarzite, occupied by both oxidized pyrites and galena 
near a green porphyrite dyke. 

The Colorado Springs mine has rich deposits of galena in the 
white Silurian limestone in close relation to dykes of diorite and 
quartz porphyry. 

The London mine in Mosquito Gulch has two strong veins of 
pyrites carrying both gold and silver, the gangue of one is quartz, 
of the other calcite. They occur in the limestone in connection 
with an intrusive bed of white porphyry. These veins stand in a 
vertical position, the beds which contain them being turned up 
abruptly against the great London fault, by whose movement the 
Archman granite rocks forming the eastern half of London Mount¬ 
ain are brought up into juxtaposition with the Silurian and Carbonif- 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cix 


erous beds at its Western point. As we go South along the 
Mosquito range the intrusive porphyries diminish in extent and 
with them also the mineral deposits. 

In the Sacramento mine rich bodies of galena and rich decom- 

\ 

posed minerals have been found in a series of pockets. Some of 
the cavities or caverns are empty, others contain sand and decom¬ 
posed pebbles of ore, and others rich deposits. These deposits are 
not easy to follow with any certainty, open fissures lead up to the 
surface of the limestone. The limestone was originally capped 
by quartz porphyry which doubtless supplied the ore. 

Placer deposits . The mountains bordering South Park owing 
to great elevation, have been much exposed to glacial action. 
An enormous amount of detrital material has thus been accumu¬ 
lated in the valleys in the form of moraines, which, when rear¬ 
ranged and concentrated by water, have formed valuable placer 
deposits. 

The first placer gold was discovered in Tarryall creek in 1859, 
and in those days produced rich results. 

Near Fairplay an immense amount of this material probably 
resulting from the influence of several glaciers, is cut by the 
Platte river exposing loose material for fifty feet. This has been 
worked for many years by sluice mining. 

At Alma a gravel bed sixty feet deep is exposed on the banks 
of the stream which is at present successfully worked by hydraulic 
mining. Gold in flakes and small nuggets has been found all 
through this mass in paying quantities, but the richest deposits 
are found in the crevices and cracks of the bed rock, which consists 
of a jointed bed of Carboniferous sandstone dipping gently. 

It is customary to rip up this sandstone for four or five feet 
until a bed of more impervious clay is reached. No gold is 
found below this second floor. The gold is collected by quick¬ 
silver thrown into the flume daily, and afterward separated from 
the quicksilver by retorting it. 

LAKE COUNTY. 

The Western boundary of this little county is theSawatch range 
of Archaean granite, penetrated by dykes of porphyry. The slope 
of the Mosquito range on the East and the hills on the North, 


cx 


GEOLOGY OF COLORADO ORE DEPOSITS. 

I 

forming the water shed between the Grand and Arkansas rivers, 
have a basis of Archaean granite*and gneiss more or less covered 
by patches and remnants of the Paleozoic formations, i. e. Silurian 
and Carboniferous, which have escaped erosion. 

Their lower position relative to corresponding beds on the 
eastern or South Park side of the Mosquito range is due in part 
to faulting, and in part to folding of the beds. 

Within these Paleozoic formations, these beds of quartzite and 
limestone, there is an enormous development of eruptive rocks 
principally quartz porphyries partially occurring as dykes but 
generally as immense intrusive sheets following the bedding plane 
of the sedimentary rocks. 

Glaciers have been at work also in this neighborhood. A. huge 
“ mer de glace ” occupied the great valley of the Arkansas to 
whose bulk numerous side glaciers contributed, these glaciers 
have carved and sculptured the mountains. In the flood period 
following the first glacial epoch a lake was formed occupying the 
head of the Arkansas valley. The stratified gravel and sand beds 
which were deposited at the bottom of this lake now form terraces 
bordering the valley of the Arkansas river. These beds, known 
as “ wash ” or placer grounds, yield gold and are open to further 
development. Leadville is the center of the mining district, the 
ores are agentiferous galena and zincblende. They are smelting 
ores. Their value is increased by their having been oxidized, 
the lead occurring as carbonate, the silver as chloride in a clayey, 
or else siliceous mass of hydrated oxides of iron and manganese. 

The ore is principally confined to the horizon of the “ blue ” 
or lower carboniferous limestone, covered by an intrusive sheet 
of “ white Leadville quartz porphyry.” The ore bodies occur not 
only at the immediate contact of these rocks but extend down in 
irregular pockets and chambers into the mass of the limestone, 
sometimes to a depth of ioo feet. Sometimes the ore completely 
replaces the limestone between two sheets of porphyry as in the 
“Col. Sellers mine,” Chrysolite, Little Pittsburg, and on Fryer Hill. 
A few ore bodies occur, carrying more gold than silver, found at 
other horizons, usually as “ gash ” veins running accross the 
stratification or. along bedding planes. Such are the “ Colorado 
Prince ” in quartzite, the Tiger and Ontario in the Weber grits of 
the Middle Carboniferous. 


GEOLOGY OF COL OR ADO ORE DEPOSITS. 


cxz 


The “ Printer Boy,” one of the oldest mines, has produced a 
good deal of gold, found as freehold associated with carbonate of 
lead and galena, passing down, as is usual in gold mines, into 
unaltered auriferous iron and copper pyrites, which occur in a body 
of quartz porphyry along a vertical cross-joint or fault plane in 
the porphyry. The gangue is a white clay resulting from decom¬ 
position of the quartz porphyry and though the clay ore is rich, 
it shows no minerals to the eye. 

The Paleozoic formations together with the intrusive porphyry 
sheets sandwiched in between them, have been compressed into 
gentle folds, and where the fold was at its greatest tension, a 
series of parallel faults have occurred having a general North and 
South direction, their uplifted side is generally to the East. 

The prevailing eruptive rock is the “white Leadville porphyry,” 
occurring generally above the blue limestone but also in places 
below it and at other horizons. 

There are also other intrusive sheets of different varieties of 
quartz porphyry. The ground is generally buried beneath a 
hundred feet of glacial moraine material, locally called “ wash.” 

The general geology of the South Park and Leadville region 
has been so elaborately traced by the labors of the U. S. Geolog¬ 
ical Survey that we cannot do better than give an abstract of 
their report in this connection : 

GEOLOGY OF THE MOSQUITO RANGE AND LEADVILLE AND SOUTH 

PARK REGION (s. F. EMMONS). 

The Rocky Mountains in Colorado consist of two parallel 
uplifts of Archaean rocks of granite, gneiss, etc., with conformable 
series of geological formations, from Cambrian to Cretaceous, 
resting upon their flanks, the later Tertiary alone being uncon- 
formable and resting on top of the upturned Cretaceous. 

The eastern uplift is called the Colorado or Front range, the 
western the Park range. The depressions between them are 
called Parks. 

The unconformity between the horizontal Tertiary and the 
uplifted Cretaceous and other beds shows that the great uplift of 
the ranges took place between the close of the Cretaceous and 
the deposition of the Tertiary. 


exit 


GEOLOGY OF COLORADO ORE DEPOSITS. 


The beds which we find uplifted and resting on the flanks of 
these ranges do not appear ever to have covered or enveloped 
the ranges of granite, but the latter formed the shore line or 
islands against which these sedimentary strata were deposited, 
and finally when the whole granite mass was uplifted the shore¬ 
line deposits were lifted with it and appear now as a fringe 
around the masses, which has suffered much since by denudation. 

The Colorado range was the most extensive of these ancient 
land masses, extending from Pike’s Peak to the boundaries of the 
State, 150 miles in length by 35 to 40 in width. North and 
South of this area it was continued by a series of islands and sub¬ 
merged reefs to the Black Hills of Dakota on the one hand and 
to the Territory of New Mexico on the other. 

THE PARKS. 

The present valleys of the North, Middle and South Parks 
were submerged in Paleozoic and Mesozoic times by the sea, 
and also in Tertiary times by fresh-water lakes. They formed a 
connected series of bays and arms of the sea and fresh-water 
lakes, as shown by the sediments of those eras still found in them. 

In Paleozoic times the outlet of the North Park was towards 
the North, of the Middle Park toward the West, and of the South 
Park to the South. 

Up to the close of the Cretaceous the North and Middle 
Parks were connected, forming a single depression. The present 
mountain barrier between the Middle and South Parks did not 
extend as far as their western boundaries, and a water connection 

1 

lay between them. 

The waters of the South Park extended westward to the flanks 
of the Sawatch range. 

In Tertiary times the Parks had been raised above the ocean 
level and were occupied by fresh-water lakes. 'Sedimentary beds 
were deposited in them, much of which have been denuded off. 
The western boundary of the Park area consisted of two distinct 
ridges or islands, forming a general line of elevation parallel with 
the Front range. 

These are the Park range proper, on the West side of the 


Sections 

i /It t Sit/"ct/e //? e // cCe / ’e/o/c /nc'/il oyC/ie, 

Ljsad'iriJJe EC £a Far^k: ftegiarz, 

o • 


:.? 

t-cs .Sects' 



SesC3.cc/ ./? /<s/c err . rj/? 


T j? Scdcoccocc. 4 AfeeJo^-cct c /C'et/a, ttn/Zt in/r tee-tccntt /./■n^/c've r ' oc S-, /*fin& . 'ncccx i/i//ie. y As j ,f(OT?<'jWz-nci^ 

/jet'O-r* to i/tf:-(/r'3ezC.A/i^itrc-/ecz.f7 c/a 1 ^ ^ rc *‘ z ^ c ' /c'J'c.' cr/ Z/s'C c C'Ccst c J 



\\l /fjj/c/2c <>, i , Sr/fact/a/TP a<// y ctefire<5£e < 



Sdett/ .S'esc/um / cfSurctStny -ncduS/ crC t/ee c?c-e-ct/ /^j/C/e/cce-ecru,/ 'Cyed-c/ir tenet- //ce r e/titncc ttfo * / ctj/ee. ACcx/scT-tt/St? f/a-ne?^- 



S/ctcns!' 



^ic(XPasc?^ y i5et>/t/rn c/ S e^dcn/CesJDCfdrie/^ ACtrftpt*C/o 7/ccnf& $ eSccuS/zs-^a^yt act-/ tc /& day, j/ccntnny 0 f fati/Scny 8; SnSjcytc^tt/ ew&tdms 


A rcAiEjnj ttrjcri/e , 6> 
C<ri<?-)p-a-c£o 7'ro 

















































































































































































1 £ ' 






































GEOLOGY OF COL OF A DO OFF DEPOSITS . 


cxm 


North Park, and the Sawatch range, now separated from the 
South Park by the Mosquito range. 

Between these was the Archaean mass of the Gore range, 
which formed, with the southern extremity of the Park range, 
the western wall of the Middle Park. 

The present boundary of the South Park on the West is the 
Mosquito range. 

Before the Cretaceous no Mosquito range existed. The rocks 
now forming its crest rested at the bottom of the sea. 

The Sawatch range is the true continuation of the Park range 
proper, as an original Archaean land mass. The Archaean land 
mass of the Sawatch in Paleozoic times was an oval island about 
70 miles long by 20 wide, surrounded by the Paleozoic seas 
laying down sediments against it. 

Through the eastern portion of this area and parallel with its 
longer axis now runs the valley of the Arkansas River, which in 
Paleozoic and Mesozoic times did not exist. 

The height of these mountain masses above the adjoining 
valleys may have been far greater then than now, since the sedi¬ 
mentary beds surrounding them and numbering some 10,000 feet 
in thickness were formed out of material washed from their 
slopes. They were, however, probably not the only land masses 
at the time from which this material may have been derived, 
other land masses may have existed and have been washed away. 
The great lava flow of the San Juan Mountains may conceal the 
remnants of a former land mass of great extent. 

The ranges were not uplifted by an upthrust from below, but 
by horizontal, tangential pressure, resulting from contraction of 
the earth’s crust, caused by the cooling of its interior ; this is 
shown by the folded character of the rock masses. The tangential 
crushing forces were applied in one case at right angles to the 
lengthwise direction of the mountain mass, in the other in a 
direction parallel with its axis, i. e., North and South. As the 
forces of contraction became stronger and the folds were pushed 
closer together, the folds broke in enormous fractures or faults of 
many thousands of feet in depth, the forces being exerted on 
either side towards the central mass. Eruptive rocks poured out 
in many cases through these fractures and added to the mountain 


CXIV 


GEOLOGY OF COL OR ADO ORE DEPOSITS. 


masses, and their ebullitions corresponded to the structural lines 
of greatest folding and faulting. Along the line of the Parks 
both earlier and later eruptions are so frequent that their out¬ 
crops form a continuous line. From the latter, the Elk Moun¬ 
tains, the head of White River, and the Elkhead Mountains in 
Wyoming, have apparently been the scenes of most violent and 
repeated eruptions during both the Mesozoic and Tertiary times. 

MOSQUITO RANGE. 

The study of this range is necessary to the understanding of 
the Leadville ore deposits, which occur on its western side. It 
comprises a length of 19 miles along the crest of the range, and 
in width includes its foothills bordering the Arkansas Valley on 
the West, and South Park on the East, a slope in one case of 7^ 
miles, and in the other of about 9 miles. All of it is about 10,000 
feet above the sea level. 

The range has a sharp single crest trending North and South. 
To the West this crest presents abrupt cliffs descending precipi¬ 
tously into great glacial amphitheatres at the head of the streams 
flowing from the range. Mts. Bross, Cameron and Lincoln con¬ 
stitute an independent uplift. The abrupt slope West of the crest 
is due to a great fault extending along its foot, by which the 
western continuation of the sedimentary beds which slope up the 
eastern spurs and cap the crest, are found at a very much lower 
elevation on the western spurs. The jagged step-like outline of 
the western spurs is due to a series of minor parallel faults and 
folds. 

The secondary uplift of Sheep Mountain on the Eastern slope 
is due to a second great fold and fault. 

The elevation of Mount Lincoln is the result of the com¬ 
bination of forces which have uplifted the Mosquito range and 
those which built up the transverse ridge separating the Middle 
from the South Park. 

The range has been sculptured by glaciers into canons and 
the Arkansas valley is covered with horizontal terraces represent¬ 
ing the distribution of material by waters, on the melting of the 
glaciers. 

In the seas of the Paleozoic and Mesozoic eras which sur- 


GEOLOGY OF COL OF A DO OFF DEPOSITS. cxv 

rounded the Sawatch islands, some 10,000 to 12,000 feet of sand¬ 
stones, conglomerates, dolomitic limestones and shales were 
deposited. Towards the close of the Cretaceous, eruptions 
occurred by which enormous masses of eruptive rock were 
intruded through the Archaean floor into the overlying sedi¬ 
mentary beds, crossing some of the beds, and then spreading out 
in immense intrusive sheets along the planes of division between 
the different strata. 

The intrusive force must have been very great, since compara¬ 
tively thin sheets of molten rock were forced continuously for 
distances of many miles between the sedimentary beds. 

That the eruptions were intermittent and continued for a long 
time is shown by the great variety of eruptive rocks found. That 
this eruptive activity preceded the great movement at the close of 
the Cretaceous which uplifced the Mosquito range as well as the 
other Rocky Mountain ranges, is proved by the folding and 
faulting of the porphyry eruptions themselves. 

In the period intervening between the close of the Cretaceous 
and the deposition of the Tertiary strata, during which the waters 
of the ocean gradually receded from the Rocky Mountain region, 
the pent-up forces of contraction in the earth’s crust, which had 
long been accumulating, found expression in dynamic movements 
of the rocky strata, pushing together from the East and the West 
the more recent stratified rocks against the relatively rigid masses 
of the Archaean land, and thus folding and crumpling the beds 
in the vicinity of the shore lines. 

The crystalline and already contorted beds of the Archaean 
doubtless received fresh crumples in this movement. 

A minor force also acted North and South, producing gentle 
lateral folds along the foothills at right angles to the trend of the 
range. These movements were not paroxysmal or sudden and 
violent, but protracted for an enormous lapse of time, and 
appear to be continued in diminished force up to the present day. 

MINERAL DEPOSITION. 

It was during the period intervening between the intrusion of 
the eruptive rocks and the dynamic movements which uplifted the 
Mosquito range, that the original deposition of metallic minerals 


jXVl 


GEOLOGY OF COLORADO ORE DEPOSITS. 


occurred in the Leadville region, probably in the form of metallic 
sulphides, though now they are found largely oxidized and in 
other combinations. They were probably derived from the erup¬ 
tive rocks themselves, and are therefore of later formation than 
them. Their having been folded and faulted with them shows 
that they must have been formed before the great Cretaceous 
uplift, and therefore that they are older than the Mosquito range 
itself. The deposits were formed by the action of percolating 
waters taking up certain ore materials in their passage through 
neighboring rocks and depositing them in a more concentrated 
form in their present position. This may have taken place while 
the sedimentary beds were still covered by the waters of the 
ocean, and the waters, therefore, may have been derived from it, 
or the area of the Mosquito range may have already emerged 
from the ocean and the waters have been estuarine. 

STRUCTURAL RESULTS OF THE UPLIFT. 

The uplift of the Mosquito range consisted of a series of 
anticlinal and synclinal folds fractured by faults. 

The crest is formed by the great Mosquito fault running North 
and South along the trend and axis of the range. 

The other great fracture is the London mine fault running in 
a south-easterly direction along the eastern spurs of the range 
coinciding with a magnificent anticlinal fold seen on Sheep 
Mountain and in Sacramento Gulch. 

On the western or Leadville side the folds, faults and cross¬ 
faults are more numerous, breaking the country up into a series 
of blocks and steps. The movement of these faults has been an 
upthrow to the East. The greatest movement is towards the 
center or Leadville region and dies out at either end North and 
.South. In the middle region the aggregate displacement is 8,000 
to 10,000 feet. 

The crests of the folds and whatever cliffs may have been 
caused originally by the displacement, have for the most part been 
planed down by erosion. The erosive forces are best seen in the 
Arkansas valley which was occupied for over ioo miles by a grand 
‘mer de glace,’ fed by numerous side glaciers from the adjacent 
ranges. There appears to be evidence in this region of two 


GEOLOGY OF COLORADO ORE DEPOSITS. 


CXVll 


glacial epochs followed severally by two intervening eras of 
warmer weather. In the former, moraines were deposited by the 
ice, in the latter, by the melting of the ice, large fresh water lakes 
occupied the broad valley of the Arkansas and have left relics of 
their former presence by extensive horizontal terraces and low 
table lands. This morainal matter together with the lake beds 
largely cover the mining area of Leadville and are called locally 
‘wash’, they also at several points afford broad gold placer 
grounds.” 

GUNNISON COUNTY 

Lies West of Chaffee County, its eastern boundary being partly 
formed by the crest of the Sawatch range. It contains both a 
plateau and a mountain region. The former is occupied by 
horizontal Cretaceous and Tertiary strata. 

Except where the Archaean granites are exposed by erosion or 
eruptive rocks have broken through the sedimentaries, there is 
not much prospect of the precious metals. Where, however, they 
do occur, vast bodies of coking, anthracite and semi-bituminous 
coals of the best quality are on hand for smelting purposes. 

The region has heretofore been retarded by the lack of trans¬ 
portation facilities; now that requisite is fully supplied by the 
Rio Grande and Union Pacific railways. 

The geology of the western slope of the Rocky Mountains 
proper differs somewhat from that of the eastern slope. In the 
latter region the strata rest usually in their natural consecutive 
order from Silurian to Tertiary upon the granite, the Silurian 
lying directly upon it, as upon a shore line. 

In the western region and slope, the Cretaceous often lies 
directly upon the Archaean and the Silurian is not found, implying 
that a land area existed over this region, raised above the Silurian, 
Carboniferous, Triassic and Jurassic seas, which were depositing 
sediment along the eastern flanks, and it was not until the 
Cretaceous era that this western area, probably by subsidence, 
was covered by seawater and marine sediments. 

The coal-forming period, which on the eastern flanks occurred 
near the close of the Cretaceous, appears to have occurred, on 
the western slope, at an earlier date in the same age. 


CXVllt 


GEOLOGY OF COLORADO ORE DEPOSITS. 


The ore deposits which in the eastern division occur in the 
Archaean and Paleozoic formations, in the western occur in the 
Mesozoic rocks as late as the Cretaceous. 

The general geological structure of the Elk Mountain region 
is that of a great “fault fold,” an anticlinal fold or arch, running 
generally with the axis of the range, broken along its crest by a 
fault. The eastern slope of the fold is gentle, but the western is 
very steep, and even overturned or inverted. 

The Carboniferous, Triassic and Jurassic have escaped erosion 
in the highest portion of the mountains, while the Cretaceous 
beds have been eroded away till they lie along the flanks. 

In the center of this fold is a mass of eruptive quartz porphyry 
and diorite, which breaks through the sedimentaries not only in 
dykes, but also in immense masses forming entire mountains, of 
which White Rock (diorite), Crested Butte and Gothic Mountains 
of quartz porphyry are typical. Some of these suggest that they 
are remnants of laccolites, those reservoirs of molten rock from 
which the strata have been removed by erosion. 

The date of these intrusive masses and eruptions is post 
Cretaceous, but their characteristics show them to be not of 
Tertiary type. The intrusion of these enormous masses of molten 
material, together with the mechanical heat engendered by the 
violent folding to which the region has been subjected, has pro¬ 
duced a widespread metamorphism of the surrounding rocks, 
including the coal which is metamorphosed into anthracite. Some 
of the limestones are changed into white marble of superior 
quality. This metamorphism, combined with other phenomena 
has made the region peculiarly favorable for metallic veins. 

These Elk Mountains are of later origin than the Sawatch 
range, and probably later than the Mosquito or Park range. 
They are apparently the youngest mountains in Colorado. 

The ore deposits in the Ruby and Irwin districts are of Cre¬ 
taceous age, as the vein and ore deposits traverse Upper Creta¬ 
ceous rocks and penetrate the Cretaceous coal horizon. 

Ore occurs at a great many localities in the Elk Mountain 
region and on the flanks of the Sawatch. 

The principal mining centers of the Elk Mountain region lie 
both in Pitkin and in Gunnison Counties, and are as below. 


GEOLOGY OF COLORADO ORE DEPOSITS . 


CXIX 


Aspen, on the northeast slope of the Elk Mountains, in the 
interval between the Elk and Sawatch ranges. 

Independence is on the West slope of the Sawatch range. 

Ruby, Gothic and others, on the South-West slope of the Elk 
Mountains. 

Pitkin and Tin-Cup, on the South-West slope of the Sawatch. 

At Independence, sulphuret ores carrying silver and gold 
occur. 

The “ Gold-Cup” mine, near Alpine Pass at Tin-Cup, occurs 
in the Carboniferous limestone similar to that at Leadville. The 
ore is argentiferous carbonate of lead and oxide of copper. 

At Irwin the “ Forest Queen ” occurs in a vein associated 
with a vertical porphyry dyke traversing the Cretaceous sand¬ 
stones. The ore is ruby silver, arsenical pyrites and sulphurets 
of silver, occurring in small crevices and fissures in the decom¬ 
posed porphyry. The gangue is an indistinctly banded quartz. 
Faults occur in the mine. 

On Copper Creek, near Gothic, a series of nearly vertical 
fissure veins traverse the eruptive diorite rocks. These veins are 
mineral-bearing, and at the Sylvanite mine have produced a great 
deal of very rich ore. Among it are very large masses of sul¬ 
phurets of silver and extraordinary specimens of native silver, in 
curly bunches resembling bunches of tow, in considerable quan¬ 
tities. Some of these silver curls are oxidized into a bright 
golden color. 


PITKIN COUNTY. 

THE ASPEN MINING REGION. 

The ore deposits of Aspen occur in the same geological hori¬ 
zon as those of Leadville, viz.: The lower Carboniferous, shown 
by the fact that the limestone enclosing the ore contains fossils 
similar to those found in the ore-bearing limestone of Leadville, 
as also by its position relative to the Cambrian quartzite and 
Archaean granite below, and the Carbonaceous shales and Weber 
erits of the Middle Carboniferous above it. 

o 

Aspen Mountain is on the risen side of a great fault which is 
clearly seen on Castle Creek on its Southern side, by which the 


cxx 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Mesozoic red beds, and other groups, have fallen several thousand 
feet from their proper position above the Paleozoic series, which 
cap the summit and sides of the mountain in which the mines 
are principally located. 

This faulting was due to the elevation of the Elk Mountains 
crushing the intervening sedimentary strata between them and 
the Sawatch range, into a series of folds which, when they had 
reached their greatest tension, broke into a series of faults. 
Besides the one great fault there were several minor faults attend¬ 
ing this movement, which are found in various parts of the 
mountain and adjacent region. Some of these faults are more 
recent than the ore deposits, as they fault them slightly. What¬ 
ever motion is visible in the mines, such as slickensides and 
smoothed walls is to be attributed to these later movements 
rather than to the production of any fissure subsequently filled by 
the ore deposits. The ore appears Jo occur as a replacement of 
limestone, the lower portion of which is dolomitic in character, 
while the upper portion is true limestone. The ore replaces both 
forms of the limestone and penetrates its mass through various 
jointing planes spreading out also at intervals between the planes 
of stratification in a more or less irregular way. The principal 
line of concentration appears to be at the point where the lime¬ 
stone becomes dolomitized. 

Overlying the limestone, but separated from it by a thick bed 
of black argillaceous shale is an enormous mass of eruptive dio- 
rite, from which it seems probable that the ores were derived 
through the medium of percolating surface waters in a somewhat 
similar way to that at Leadville. 

The ores are fine-grained galena, rich in silver. A good deal 
of silver sulphurets, polybasite, and decomposition products of 
these. 

As we published the results of our examination of the district 
in last year’s report, we will not repeat the details here, but as the 
region has so rapidly developed in importance since then, an 
importance which is daily increasing, the results of a subsequent 
examination by Mr. S. F. Emmons of the Geological Survey, 
read before the Scientific Society of Denver, will be of interest, 
and we give an epitome of it here, with the greater pleasure as 



Viewed from 

SMUGGLER HILL 


SJUreft. 






JLdLk lJfrHiJ'W'TS 


s OPR»s M* 


I Connernara 

■)'* 

<56PB/lt 


ToupMyny 


rranKli 


Stggfe 


Veteran Tunn»l 










































































©r»_ 
































GEOLOGY OF COLORADO ORE DEPOSITS. 


CXXl 


while confirming many of the published views of the writer of 
this treatise and correcting others, it adds also a great deal of 
fresh material of great value to those interested in the Aspen 
mines. 

ABSTRACT OF PAPER ON THE ASPEN MINING REGION, 

BY S. F. EMMONS. 

“ The Aspen mining region is related to that of Leadville. 
Each is on the shore line of the old Archaean island of the 
Sawatch, one on the East, the other on the West, and about 
opposite one another, (though about fifty miles apart.) 

The ore deposits occur in the same general horizon as at 
Leadville, viz., the Lower Carboniferous limestone. 

Both are regions of intense dynamic disturbances, which have 
been accompanied by immense intrusions of igneous rock through 
the sedimentary strata. 

The process of ore deposition in either region has been an 
actual replacement of the country rock by vein material, and 
where open cavities occur they are found to be of later formation 
than the original deposition of ore. 

At Aspen the ore is not found in actual contact with the 

eruptive bodies, as is generally the case at Leadville, and the 

country rock, instead of being entirely dolomite, is only partially 

so, and whereas at Leadville there is reason to assume that it 

was originally deposited as a dolomite, at Aspen there is some 

reason for thinking that the dolomitization may have been in 

part, at any rate, a secondary process, entirely subsequent to tfce 

» 

deposition of the limestone.” 

“The mines of Aspen are situated in Paleozoic strata reclining 
upon the slope of a narrovv ridged mountain forming a granite 
spur en echelon with the Sawatch range. 

The strip of country in the vicinity of Aspen constitutes the 
dividing line between the two distinct uplifts of the Sawatch 
range on the East and of the .Elk Mountains on the West, and 
has been affected successively by the dynamic movements accom¬ 
panying each upheaval. 

The Sawatch upheaval was a gradual elevation of this moun¬ 
tain mass, resulting from a gradual subsidence of the adjoining 


CXXll 


GEOLOGY OF COLORADO ORE DEPOSITS. 


sea bottoms, which caused the sedimentary beds deposited in 
those sea bottoms to slope up at varying angles all along the 
ancient shore line toward the central mass of the Archaean island. 

The Elk Mountain range, which extends to the West and 
South of this region, was upheaved later than the Sawatch, with 
greater violence and eruptive energy, and the upheaval was 
accompanied by enormous intrusions of eruptive rock which 
were forced into the sedimentary strata already shattered by the 
forces of upheaval, in great ‘laccolites,’ or solid masses, and 
spread out through them in every direction in the form of dykes 
and intrusive sheets. The surface exposures of these igneous 
bodies cover areas of twenty-five to thirty square miles, and 
their extension below the surface is doubtless very much greater. 

The intrusion of such enormous masses of foreign matter 
must not only have greatly disturbed the beds within the region 
of upheaval, but also have so expanded the volume of the earth’s 
crust in this area as to cause a severe lateral pressure in the 
adjoining region. That adjoining region was Aspen and its 
neighborhood. 

It would be just in the strip of sedimentary beds along the 
Aspen Mountain ridge, which is backed by a projecting point of 
the unyielding Sawatch Archaean that this compression would 
be most severely felt, the Sawatch granite mass acting as a point 
of resistance against the intense lateral compression caused by 
the younger Elk Mountain uplift. 

The sedimentary beds resting against the Archaean correspond 
generally, with slight differences, to those in the South Park 
and Leadville region in a similar position. 

STRATIGRAPHY OF ASPEN. 

The latter were deposited in a partially enclosed bay, now 
constituting the South Park basin, the former on the West side 
of the Archaean island in a wider and deeper sea, and on this 
western slope the beds are generally much thicker than those of 
corresponding geological horizons on the East. 

1. The horizons represented are the Upper Cambrian quartz¬ 
ites, 200 feet, resting on the Archaean granite. 

2. Silurian silicious limestones and quartzites, 340 feet. 


GEOLOGY OF COLORADO ORE DEPOSITS. cxxiii 

3. Darker limestones, rusty brown and dolomitic at base, blue 
compact and pure on top, 240 feet. (These are Lower Carbon¬ 
iferous.) 

4. Carboniferous clays and shales and thin bedded limestones, 
425 feet. These belong to the Weber grits (Middle Carbon¬ 
iferous). 

5. A series of variegated green and red sandstones, clays 
and shales, some limestones and red sandstones of the Upper 
Carboniferous. 

6. Heavy bedded red sandstones (Triassic). 

Above these again are several thousand feet of Cretaceous 
strata, up to the base of the Laramie coal beds. (The Creta¬ 
ceous, however, and the Jurassic do not rest immediately in any 
case upon the granite.) 

Dionte. On Aspen Mountain is a bed of white porphyry 
(diorite) in the black shales, 60 to 100 feet above the top of the 
blue limestone. It is 260 feet thick on the slope back of town, 
but thickens considerably to the South, and is traceable to Ash¬ 
croft.” [It appears to extend also across the valley of Roaring 
Fork to Smuggler Mountain. Small intrusive sheets also occur 
in the lower quartzites near the point of Aspen Mountain and on 
the East face of Richmond Hill.] 

“As affected by the Sawatch upheaval, these beds wrap around 
the Archaean mass resting against or dipping away from it at 
varying angles. 

The quartzites and limestones cross the valley of Roaring 
Fork from Smuggler Mountain to Aspen Mountain, striking 
North-East and South-West, dipping North-West. The angle 
of dip is about 45 °, varying from a minimum of 30° to a maxi¬ 
mum of 6o° in ‘ flats ’ and ‘steeps.’ 

At the upper end of Spar Ridge, blue limestones change in 
strike from North-East to North, bending to the South till they 
reach Ashcroft, the westward dip shallowing nearly to a horizontal 
at the head of Spar Gulch and steepening again to 45 0 near 
Ashcroft. 

In the hills forming the East bank of Roaring Fork valley, 
from Smuggler Mountain Northwestward is a continuous con¬ 
formable series of beds from Cambrian to Cretaceous dipping 


CXXIV 


GEOLOGY OF COLORADO ORE DEPOSITS. 


Northwest. Were this region affected by the Sawatch upheaval 
alone, we should expect to find this same series sweeping contin¬ 
uously around and resting cotnformably upon the flanks of the 
lower Paleozoic strata which form the crest of the ridge ,from 
Aspen to Ashcroft. Instead of this, on the steep West slope of 
Aspen Mountain, towards Castle Creek, we find, now the blue 
limestone, now the Cambrian quartzites and again the Archaean 
granite, abutting against the Triassic beds, and going northward 
along the East slope of the Mountain back of Aspen City. 
After passing geologically upwards through blue limestone, black 
shales, porphyry and black shales again, we find the series 
repeated at the point of the ridge from granite up, to blue lime¬ 
stone again, the latter beds lying in great slabs against its North¬ 
ern ends, striking East and dipping about 6o° to the North. 

This is the result not of mere folding but the extreme 
compression resulting in faulting. 

There is not only one great fault, but several smaller parallel 
ones. This compression proceeded from the upheaval of the Elk 
Mountains crowding the sedimentary beds against the unyielding 
Sawatch Archaean mass, so that along its edge they have been 
broken across and shoved up past each other. 

CASTLE CREEK FAULT. 

The line of the principal fault is shown by its movement 
bringing the red sandstones in juxtaposition to the limestones, 
quartzites and Archaean rocks on the East. The minor faults are 
more obscured by debris. The main fault is visible around the 
point of Aspen Mountain, where Castle Creek cuts into its 

northern foot. Vertical red sandstones striking North and South 

* 

appear parallel to the fault plane. These adjoin the steeply 
upturned quartzites which strike East and West across the 
northern end of the ridge. The fault runs for several miles 
southward along the foot of the hill, parallel with the bed of the 
creek, gradually rising higher on the slope. On the West side of 
the fault the red beds stand either vertical or dipping slightly 
eastward. 

In the hills on the West side of Castle Creek the same beds 
are nearly horizontal, or dip io° to 20° to the North down the 


I 


GEOLOGY OF COL OF ABO OFF DEPOSITS. cxxv 

creek. The beds exposed are successively lower as we ascend 
the creek. At Queens Gulch there is a decided dip eastward of 
the red beds 15 0 from a vertical. To connect the vertical beds 
with the horizontal on the opposit side of the creek would involve 
an S shaped synclinal. Such a fold is evidence of intense compres¬ 
sion accompanying the faulting, sufficient to double together these 
heavy sandstones as closely as one folds sheets of paper. Queens 
Butte about two miles below Aspen is a good example of this 
overturned S fold, the Jurassic beds lying on top of the over¬ 
turned Cretaceous. 

This butte is on the same line of fault and marks its 
continuation in that direction. 

In Ophir Gulch the line of the fault is well marked by an 
outcrop of granite in the bed of the gulch adjoining the sharply 
upturned red sandstones. A tunnel has been run in on it, and 
the fault plane of the granite wall dips 45 0 to the East. 

The fault there is a reversed fault, because the upward move¬ 
ment was in the hanging wall contrary to the usual law of faults 
by which the foot wall rises. The plane of the Castle Creek fault 
has an eastward dip, instead of a westward one, implied also by 
the fact of the beds immediately adjoining the fault on the West 
often dipping East also. 

Beds West of the faults were more plastic than the older 
ones now adjoining it on the East. The former not being fractured, 
the latter being broken by many minor faults parallel to the main 
fault with no evidence of such closely compressed folding as 
exists in the former. 

In Queens Gulch white quartzite is the first outcrop East of 
the fault, then over-lying brown limestone, a gap, and then 
quartzite dipping 45 0 West, granite below them in tunnel. One 
thousand feet above this are the Queens Cliff outcrops of blue 
limestone and brown limestone forming the southern point of 
Aspen Mountain. 

On the ridge running West from Queens Cliff between Ophir 
and Queens Gulches are three minor faults West of the main 
fault One mine shaft here had a limestone East wall and a red 
sandstone West wall. Another shaft in a fissure, had porphyry 
on the East and limestone on the West. Another had limestone 


CXXVl 


GEOLOGY OF COLORADO ORE DEPOSITS. 


one side and granite the other. The latter had crossed the fault 
line and was drifting hopelessly into the Archaean mass. 

On the summit of Queens Gulch at Queens Cliff is an expo¬ 
sure of several hundred feet of blue and brown limestone dipping 
15° West. On the ridge, East of this, across the head of Spar 
Gulch on Ajax Hill the same series is repeated, separated by a 
slight fault on the line of Spar Gulch. Following down the East 
slope of this hill we cross successively the lower limestones and 
Cambrian quarzites striking North and South dipping gently 
West, and found resting upon the Archaean at the base of the first 
steep slope of the ridge, 500 feet below the summit. 

NORTHERN PORTION OF ASPEN MOUNTAIN. 

For some distance North of Queens Cliff limestone is the 
cap rock of the ridge, the overlying porphyry having been eroded 
off. On the round-topped hill at the head of Ophir Gulch on 
whose East face is the “Campbird” Mine, several hundred feet of 
porphyry overly the limestone. 

Its sudden appearance is accounted for by a fault crossing its 
Southern end running West of North with upthrow on the South. 
Northward from here as far as the head of Keno Gulch porphyry 
forms the whole upper portion of the ridge. Below it along the 
West wall of Spar Gulch the limestones and their ore-bearing 
zone are traced by occasional out-crops, and by the workings of 
many mines. 

The line between porphyry and limestone descends to the 
North till the change of strike from North to Northeast comes in, 
where the bend causes a steepening of the strata. 

The blue limestone of Spar ridge runs diagonally across the 
course of Spar Gulch closing it into a very narrow ravine in its 
lower portion, and finally crossing it at the bottom, passes over 
the point of the main ridge to the East of it, to disappear beneath 
the valley of Roaring Fork. 

The overlying porphyry also bends around to the Northeast 
conformably and forms the ridge bounding Valejo Gulch on the 
North, on the sloping crest of which several shafts have been 
sunk. 

To the North, beneath the surface, on the East slopes of Aspen 


GEOLOGY OF COLORADO ORE DEPOSITS. 


CXXVll 


Mountain several hundred feet of dark limestones and Carbona¬ 
ceous shales rest conformably on the porphyry sheet though near 
the surface they have been eroded off. 

The beds follow normally the outline of the Archaean, not 
broken by any serious faults. 

Minor faults following the direction of the Castle Creek fault 
will be found like that which runs across Spar ridge above the 
Durant cliff, extending across the Bonny-Bell ground on the 
.South and the Aspen on the North. 

From the head of Keno Gulch North, the ridge of Aspen 
Mountain is set off ert echolon to the West of the main ridge, as 
the whole body of the mountain is set off from the main ridge at 
Queens Cliff. The highest and most Southern point of this ridge 
overlooking the head of Keno Gulch, (‘Acquisition Hill,’) is 
separated from the main ridge by a V-shaped depression running 
North and South. 

Brown limestone, granite and white quartzite appear, the 
former at the Southern end and running along the Eastern face in 
a northerly direction. Granite adjoins it on the West and 
the quartzite rests on granite on the North and West. 

Faulting has brought the limestone and granite into juxtapo¬ 
sition. The fault plane is cut in the Acquisition mine, whose 
tunnel passes from the brown limestone into granite at 300 feet. 
Following the ridge North, beds of limestone and quartzite appear 
striking North and South, with steep dips. About half way to 
the North point granite comes in, forming the crest of the ridge 
as far as where the successive great slab-like masses of quartzite 
and limestone rest against it, and dipping steeply North, form the 
extreme point of the mountain. 

On the South and East of this mass of granite, the beds are 
broken by a series of minor faults, running North and South. 
Near the Pioneer, in some tunnels, vertical faults and slip planes 
running North and South occur, the striations indicating an 
upward and downward movement at an angle of 6o° to the hori¬ 
zon, towards the North, or in a reversed direction downward 
6o° towards the South. The beds North of the granite have an 
East and West strike, but tend to wrap around the granite body, 
as in the East they curve in strike to the southward. 




cxxviii GEOLOGY OF COLORADO ORE DEPOSITS. 

The structure of this ridge is as follows: By the movement 
of Castle Creek fault this body of granite and the strata resting 
on its North side were dragged bodily upwards from their nor¬ 
mal position on the downward dip of the beds out-cropping in 
Spar Gulch, and with a relatively greater movement of displace¬ 
ment than the rest of the region, since they must have been 
lower down originally. 

The upward movement was relatively greater immediately 
adjoining the fault than at some little distance to the East, and 
thus the West end of these uplifted beds was carried further 
upward and northward than the East end. Their strike was 
shifted from North-East to East and a little South of East. 

In the intermediate region to the South-East between the 
granite beds and the normally dipping beds of Vallejo Gulch, 
which is farther away from the fault plane, the beds were dragged 
up on the flanks of this upward moving granite body, not in a 
single mass like the strata to the North, but holding back as it 
were, sloping up against it at steep angles, and slipping back 
along minor fault planes. 

As the limestone on the steep side by the Pioneer mine dip 
East, an anticlinal fold over the granite, and a synclinal fold 
or basin between it and Vallejo Gulch has been assumed to exist. 
There may have been an abortive attempt to form such folds, 
but the space was too limited for their free development and 
they were fractured before the folding was completed. There 
are no continuous unbroken curves representing folds except 
perhaps in the re-entering angle of the hill formed by the upper 
part of Pioneer Gulch. 

The New York tunnel, 1,000 feet above the town, runs 1,100 
feet into the hill in a direction South 20° West. It crosses the 
strata at right angles. 

These have a North-Westerly instead of a North-Easterly 
strike. It passes through ioo feet wash, 585 feet conformable 
limestone, shales and included porphyry, before reaching the top 
of the blue limestone. 

It is possible, though improbable, that a horizontal drift run¬ 
ning South-East along a given bed, such as the blue limestone, 
would make a continuous curve to the North-East unbroken by 



.. . . 

' ° -J j\*' „ * # ** /# * * « * 

« « r » *y \ •**•*•*** « 

.,•/'• /i : . 

„ ' '/c'uia rite ,' Porphyry 

•• \J J » * *, 

o * • * „ . - - 

» 9 7 7 h o • * , *■ 

°S • ,/ « . . > c ‘ ^ * 

Landslide Drift and Debris 




ARCHAAN CAMBRIAN SILURIAN CARBONIFEROUS’ 

Gx^rLcrrzs cry**_u?£sCrzZJ~/Crz^ _J5^jeep£> 



K.W. 


CARBONIFEROUS & SILURIAN 

M/sddce- Q'ZoMeJ'&rfowf* 


A-JR C H V=5_ A. TM 


CAMBRIAN , SILURIAN & LOWER CARBONIFEROUS . T RIAS g j 


C ? 


>uia.kfrtlUQr3!i!i°U iJv "nr ~f=!^f fj*!: y p- : r fti; 


7(A^y0^ <2&€zn£' 


^ U R A SS IC c! FL 2“ T* _A c f? o rr 

Ao^er^oowr- zCf,^-JuratrU Dcvkci^ CrcJ*, ^fojrAMi A'aS^ 

**£a*&rrue 6 r &£4 






























VIA jJ\HO B A 

























CXIX 


GEOLOGY OF COLORADO ORE DEPOSITS. 

\ 

a fault before reaching the out-crop of the Spar Gulch, and so 
prove the existence of a synclinal basin under the northern por¬ 
tion of the porphyry. Of continuous folds North of this, there 
is no possibility, and while the Spar Ridge limestone stretches 
across Roaring Fork Valley to Smuggler Hill, and ore bodies 
may be found beneath the valley, the same continuity cannot be 
expected in the limestone beds of the northern point of the 
mountain. The beds in a corresponding position to them on the 
East side of the valley belong to a higher geological horizon, 
hence somewhere in the valley between they must be cut off by 
faults, probably nearer to Aspen Mountain than to the other side 
of the valley. 

% 

ORE-BEARING HORIZON SUPPOSED TO BE AT CONTACT OF BLUE LIME- 

\\ 

STONE AND UNDERLYING DOLOMITE OR BROWN LIMESTONE. 

The blue limestone is compact, homogeneous and pure carbon¬ 
ate of lime, when crystalline its crystals are larger than those of 
the dolomite. The brown limestone, when unaltered, is of a dark 
blue gray, finely crystalline, finely granulated, and traversed in 
every direction by a net work of minute veins containing iron 
salts which when oxidized color the surface a rusty brown. 

The oxidization along these minute veins makes it break easily 
into dice-shaped fragments giving the rock a ‘crackly’ structure 
hence its name of ‘short lime.’ The rock has not been crushed 
or brecciated, it is a true dolomite. 

The thickness of the blue limestone is from 120 to 150 feet. 

On the cliff on Spar ridge above the Durant Mine, six lenticular 
seams of dolomite are included in the blue limestone, wheathering 
more rapidly than the blue, they produce indentations in the cliff. 

On Smuggler Mountain the presence of the blue limestone is 
not clear. It is doubtful if the ‘contact’ is one and the same 
geological horizon throughout the region. 

From a few fossils and lithological structure, both blue and 
brown limestone belong to the same horizon, the Lower Carbon¬ 
iferous. The ore was not deposited in a fissure formed by the 
movement of the blue limestone over the brown dolomite, if it 
had been, striated surfaces and crushed material would be found 


cxxx 


GEOLOGY OF COLORADO ORE DEPOSITS. 


along the plane of movement. Such slickensides and structures 
as are found in portions of the Spar and other mines are due to 
movements after the ore was deposited.” 

Ore Distrifaition .—The outlines of the ore bodies cannot be 
detected by the eye owing to the gradual transition from ore to 
country rock. 

The ore is not confined to the brown dolomite below the 
contact but several ore bodies extend 20 or 30 feet above this 
contact into the blue limestone and in some cases follow the lines 
of cross-fracture entirely across the blue limestone. 

This ore is not confined, either, to a definite plane or contact 
between two dissimilar beds of limestone and dolomite from 
which its solutions have eaten into the underlying dolomite, for 
in the first place there is not one single contact, but many, and if 
this so-called “contact” constitutes an essential condition of ore 
deposition, there is no reason why it should be confined to the 
one and not found in the others where the rocks have the same 
composition. Again, ore-bearing solutions are not likely to eat 
upwards for any great distance from the contact plane if they 
entered the beds along this plane. 

This contact plane is well defined on Spar ridge and continues 
down with the dip in the underground workings, but ore bodies 
occur both above and below it. 

The rock thus mineralized is dolomite in most cases, but is 
none the less above the true bedding plane called the contact. 

In other parts there has been fracturing across the beds as 
shown by a vertical breccia of limestone fragments with a cement 
of iron oxide and manganese. 

Over the ore bodies are lines of open cavities following the 
lines of cross-fracture through which the ore solutions passed 
which deposited the ore bodies. These caves are now being 
hollowed out by water descending from the surface dissolving the 
limestone in the roof and flowing off along the floor depositing a 
mud of silica alumina, lime, magnesia and iron oxide. 

Hence this contact is not necessarily the only ore channel of . 
the district, and other channels may be sought for. 

Portions of the ore bodies have been formed by solutions per¬ 
colating through cross-fractures and spreading out between the 
parallel bedding planes. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


CXXXl 


This would happen if these solutions derived their metals 
from the overlying porphyry, for it is separated from the limestone 
by argillaceous shales which would be impervious unless fractured 
across the bedding. The analysis of the lime mud at bottom of 
the cave shows by its preponderance of alkalies, which do not 
exist in the composition of either brown or blue limestone, that 
the waters dissolving it came from the porphyry. The waters 
brought both alkalies and silica from the porphyry, and probably 
the iron and baryta. 

DOLOMITIZATION. 

This is a secondary process upon the blue limestone by mag¬ 
nesian waters, which is proved by irregular tongues of dolomite 
extending up into and across the blue limestone. The lenticular 
bodies in the Durant cliff point to the same fact. The crackly 
structure of the brown lime results from the replacement of a 
molecule of lime by a molecule of magnesia, involving also a 
contraction in volume of the rock itself, which would cause it to 
separate in angular fragments, the intersections filled by material 
more soluble than the rock itself. 

The magnesian waters may have been connected with those 
which brought in the vein materials. 

In the ore bodies the partially mineralized rock on the bor¬ 
ders of the ore is changed to dolomite, hence dolomitization either 
preceded or accompanied ore deposition. 

Mr. Emmons suggests as probabilities only, that the porphyry 
intrusion preceded the faulting. 

That the ore deposit followed the intrusion of porphyry and 
also the principal faulting movements. 

That small movements have taken place in recent times both 
in the strata and contained ore bodies since the oxidation of the 
latter. That at the time of the great faulting, the beds may not 
have attained entirely the present position. 

THE SAN JUAN REGION 

This region, which embraces San Juan, Hinsdale, Ouray, La 
Plata, and part of Rio Grande and Conejos Counties, takes its 
name from the San Juan Mountains, a lofty, irregular mountain 


CXXXll 


GEOLOGY OF COL OF A BO OFF DEPOSITS. 


mass of a North-West trend, composed almost entirely of pro¬ 
digious flows of lava, emanating in all probability from a series of 
dykes concealed underneath the flows. These horizontal flows 
have buried under their mass the primitive granite which is 
occasionally to be found peeping out from underneath it at the 
bottom of the profound canons, from whose depths you can look 
up at a vertical section of from 2,000 to 3,000 feet of lava lying 
layer upon layer of different colors. Some of these eruptive 
rocks consist of enormous thicknesses of volcanic “breccia” or 
conglomerate, which have a leadish gray appearance at a distance, 
but on nearer inspection the fragments composing them partake 
of an olive green or lilac color. There appear to be two sets of 
these eruptive rocks, one of older date than the other. The 
breccias belonging to a porphyritic or dioritic type are locally 
penetrated by dykes or covered by flows of newer eruptions, 
such as rhyolites, andesites and basalts. While some are Tertiary 
eruptives, others are of older date, and it is in these latter, par¬ 
ticularly in the breccias, that the mineral veins more especially 
occur. 

In certain portions of this region, besides the granite, sedi¬ 
mentary rocks of Carboniferous age, such as limestones and 
quartzites, and red sandstones of Triassic age, appear from under 
the prodigious lava covering. 

They are generally more or less uplifted, while the lavas rest 
upon them horizontally. It is apparent then, that the eruptions 
occurred after the Triassic, and also after the underlying strata 
had been uplifted into something like mountain forms. 

At the head of Uncompahgre River, near Ouray, Paleozoic 
limestones and quartzites, and Triassic red sandstones, rest on 
the granite, and dip to the North-West under the Cretaceous 
formations covering the Counties of Ouray and La Plata, which 
belong to the Colorado plateau region. In the Southern part of 
San Juan County the same feature is observed. 

Ore Deposits .—Probably few regions in the world are trav¬ 
ersed by so many and such large veins. Immense vertical veins 
of a hard bluish quartz traverse the eruptive rocks. Their out¬ 
crops project like walls from the surface, or run down either side 
of the profound canons for several thousands of feet, and though 


GEOLOGY OF COLORADO ORE DEPOSITS. cxxxiii 

they penetrate both older and younger eruptive sheets, the ore 
bodies are most productive in the older eruptions, especially the 
brecciated rocks. Veins also occur in the underlying granite and 
gneiss. 

At Rico deposits occur between the bedding planes of Car¬ 
boniferous limestones at contact with sheets of intrusive igneous 
rocks. The deposits are mainly argentiferous. Both gold and 
silver, however, occur in the lodes. Rich gold placer deposits 
are found near San Miguel. 

The minerals are principally argentiferous galena, gray copper 
and bismuthinite, with ruby silver, native silver and 
zinc-blende. Molybdenite is not uncommon. Barite sometimes 
forms the gangue in place of the hard chalcedonic quartz. The 
veins often have a banded structure, and sometimes a brecciated 
one. In many cases one or both walls are not clearly defined, 
and a portion of the vein material is decomposed country rock. 

There are two sets of veins traversing the region, one having 
a North-West by South-East, the other a North-East by South- 
West direction, thus cutting one another diagonally. The North- 
West is the commoner in direction. 

CUSTER COUNTY. 

“Comprises the Wet Mountain valley lying between the Wet 
Mountains or Greenhorn range on the East and the North end 
of the Sangre de Christo range on the West. 

The Greenhorn range is a continuation of the front or Colo- 
rado range of Archaean granite with Mesozoic formations resting 
against its eastern base. 

The lofty Sangre de Christo range is a continuation of the 
Mosquito or Park range and its geological structure, but little 
known, is probably similar, viz., Paleozoic quartzites and lime¬ 
stones, resting upon granite and traversed by dykes, of porphyries 
and other eruptive rocks. 

The principal mines are near Silver Cliff and Rosita, an area 
of ten miles by six. The underlying Archaean is broken through 
and covered by eruptive rocks consisting of diabase, a heavy dark 
rock, andesite, and rhyolite, which outcrop at Silver Cliff and 


cxxxiv GEOLOGY OF COLORADO ORE DEPOSITS. 


Rosita. Silver Cliff city is on the open plain near a ‘mesa’ 
ridge on whose cliff face the silver deposits are developed in the 
‘ Racine Boy ’ mine. The rock of this cliff is a light pinkish 
rhyolite, showing the laminated fluidal structure peculiar to this 
variety of lava. A black glassy variety of rhyolite also occurs. 

Outcrops of granite are found on the plains between Silver 
Cliff and Rosita, implying that the rhyolite rests on under-lying 
Archaean. Two miles North in the Blue Mountains, is the ‘Bull 
Domingo’ mine. 

The ore deposits are peculiar. The ‘ Bassick ’ and ‘ Bull 
Domingo ” situated near the northern limits of the eruptive rocks, 
are among the most remarkable mines. 

The peculiar feature of both these mines is that the ore is 
found in large bodies without any definite boundary, forming a 
coating on irregularly rounded fragments of the adjacent country 
rock. The popular theory that the mines are old craters or sol- 
fataric openings in which the fragments of rock have been tossed 
about rounded by attrition and coated by metallic vapors and solu¬ 
tions, does not seem to be borne out by careful examination of 
the neighborhood, according to Emmons. 

The country rock of the ‘Bull Domingo’ is a hornblendic 
gneiss of Archaean age. 

The ore, principally agentiferous galena, forms a regular semi¬ 
crystalline coating from one-eighth to one-fourth inch thick 
around boulders and pebbles of rock, and fills the interstices 
between them. These pebbles are not in direct contact one with 
the other, but are separated by the metallic coating belonging to 
each individual pebble. 

The galena is covered by a second botryoidal coating of a 
silicious nature. The deposit is from forty to sixty feet wide and 
strikes in a northwesterly direction. The country rock of the 
‘ Bassick’ appears to be a ‘Breccia’ of volcanic pebbles and the 
ore to be a replacement of the ‘Matrix.’ 

The Bassick deposit is an irregular oval opening from twenty 
to ioo feet in width, occurring vertically to the present depth of 
development of Boo feet, an oval well, as it were, in the rocks. 

The fragments of rock filling this opening, vary in size from 
two feet to the smallest dimensions. They are rarely in contact 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cxxxv 


with one another while the metallic coatings surrounding them 
touch one another. The size of the fragments as well as the 
quantity of ore present decreases from the center outward without 
any definite limit having been determined. 

In the metallic coating there is a series of concentric layers 
the innermost and thinnest consists of sulphide of lead, antimony 
and zinc, assaying sixty ounces silver and one to three ounces 
gold to ton. 

A second coating lighter in color contains more lead, silver 
and gold. 

The third shell is zinc-blende, carrying sixty to 100 ounces 
silver, fifteen to fifty ounces gold, with a good deal of iron and 
some copper. 

The fourth shell is of copper pyrites, carrying fifty to ioo 
ounces gold and silver. These layers are not always constant or 
complete, they have a crystalline structure; the remaining inter¬ 
sections between the pebbles are filled with Kaolin. 

Small fragments of charcoal are found in cavities between 
the boulders toward the outer edges of the ore body and near 
the water level. These are sometimes partially mineralized and 
at others are perfectly unaltered and retain the woody structure. 

The greatest depth at which they have been found is 765 feet 
from the surface. 

Silicate of zinc, gray copper, free gold and tellurides of silver 
and gold are also found in small quantities in the mine. 

The ‘ Racine Boy,’ near Silver Cliff, is an irregular impreg¬ 
nation of the country rock. Chloride of silver occurs in it, the 
ore is low grade but occurs in great quantities. 

From the Gem Mine a rich nickel ore has been obtained. 

HUERFANO COUNTY. 

Ore occurs in the Spanish Peaks. The ore is galena, pyrite 
and gray copper, occuring in a gangue of quartz in the joints of 
the porphyry. In the ‘Monarch’ mine the veins are in the 
porphyry which composes these majestic eruptive Mountains 
whose history is that of a vast number of dykes emanating from 
a grand center of eruption which constitutes the core of these 
peaks.” 


cxxxvi GEOLOGY OF COLORADO ORE DEPOSITS. 


CONCLUDING REMARKS. 

These examples we have selected from the principal mining 
districts of Colorado to illustrate the principles in the preceding 
part of this work, afford us good types of the geological 
formations in which the ores are principally found as well as their 
typical mode of occurrence, viz: The Archaean granitic series 
with its fissure veins, the typical volcanic region of San Juan, in 
which fissure veins also occur in enormous masses of eruptive 
rock only, and the sedimentary series, such as limestones and 
quartzites, penetrated by intrusive eruptive rocks in which the ore 
occurs in bedded deposits or pockets. 

There has been and may still exist a prevailing prejudice in 
favor of fissure veins on account of their supposed steady reliability 
to great depths, and against bedded deposits or “blanket veins” 
as they are sometimes called especially when they occur in lime¬ 
stone, on account of their supposed uncertain continuity for any 
great distance. 

The developments of Leadville and Aspen have shown this 
prejudice to be unfounded, at least in Colorado, and haveal most 
overturned the scale in favor of bedded deposits in limestone, 
since the riches derived through a few years from these formations 
have far exceeded all that has been obtained from the fissure veins 
of Colorado in granitic rocks in the whole course of Colorado 
history. It is true that sometimes ore deposits occur in a series 
of almost isolated and disconnected “chambers” or “pockets” 
and that much labor and expense is spent in hunting for a second 
pocket after the first has been exhausted, but they are the excep¬ 
tions in Colorado rather than the rule, and quite the same thing 
not unfrequently occurs in the more popular fissure veins. A 
‘pinch’ or a practically barren interval of an unknown length may 
occur between one productive part of a fissure vein and another. 

In the Leadville ore deposits the ore not only occurs as a 
broad sheet or blanket between the overlying porphyry and the 
limestone, but also descends at intervals in large and irregular 
pockets into the mass of the limestone itself. The same occurs 
at Aspen also. There has been far less difficulty in both these 
camps of following down these blanket deposits with occasional 


GEOLOGY OF COLORADO ORE DEPOSITS. cxxxvii 


pinches and widenings, and pockets in comparatively soft lime¬ 
stone gangue than in pursuing the steep downward course of 
a fissure vein in granite or eruptive rock with its hard gangue of 
quartz. 

As to the continuity and reliable lasting powers of these 
so-called bedded blanket deposits so far as we can judge from 
those at Leadville which have been worked now for upwards of 
ten years, there seems nothing against there lasting for centuries, 
or to the limit of depth to which mining operations can be carried. 

The ores in limestone, from their oxidized character, are 
easier treated at the smelter as a rule than those found in fissure 
veins. 

As for degree of richnesss, we have but to look at the late 
extraordinary rich developments at Aspen to show that bedded 
deposits in limestone can equal and far surpass in richness and 
quantity the average fissure veins in granite rocks, so that the 
answer of an experienced mining man to the question whether 
he would rather have a fissure vein in granite or a bedded deposit 
in limestone, had some reason in it when he replied, “Give me a 
limestone deposit, every time.” We do not underrate fissure 
veins, but we think they have been overrated to the disparage¬ 
ment of other forms of mineral occurrence quite their equals, and 
often their superiors. 

With regard to fissure veins, from what we have said it will 
appear that in Colorado, at least, we have very few that will 
answer to the orthodox type, as represented in some of our text 
books, viz., a once wide open fissure with well defined walls that 
has been gradually filled by mineral solutions ascending from 
heated depths below. 

We do not think the original fissures were wide, or at least 
so wide as some of the fissure veins now are, such as fifty to 
one hundred feet in width, but that they were rather narrow 
cracks, such as produced by faulting and jointing, or lines ol 
weakness between stratification planes which were worked upon 
by solutions oozing from the adjacent country rock, robbing that 
rock of the metal elements minutely contained in its constit¬ 
uent minerals, and redepositing them in various combinations in 
a more available and crystalline form in the fissure or line of 


cxxxviii GEOLOGY OF COLORADO ORE DEPOSITS. 


weakness. The great width of some of our veins we attribute 
rather to the altering, corroding power of these solutions on the 
country rock than to the original width of the fissure. 

The fissures or fault lines of some of the profound faults 
in our mountains do not present a wide open fissure abyss, but 
narrow cracks sometimes almost welded together again by heat 
and pressure; the adjacent rock in the immediate proximity of 
the fault is also much broken, cracked and fractured. Upon just 
such a line of weakness and of broken rock, mineral solutions 
working, would dissolve, alter and replace portions of the broken 
rock constituting a wide vein whilst other portions would be left 
unsubstituted and constitute a breccia, or the solutions working 
around large fragments would present us with the phenomena of 
those horses and split veins we see so well represented in the 
fissure veins of San Juan and elsewhere. 

The medium for filling these fissures with veinstone and metal 
was in all cases water, probably heated and charged with various 
alkalies and salts. 

The favorite circumstances for rich ore occurrence in Colo¬ 
rado are in what are called “contacts.” The ore body, lying 
between two rocks of a different character, usually between an 
eruptive porphyry and some other sedimentary metamorphic or 
non-eruptive rock. These “contacts” may occur both in connec¬ 
tion with fissure veins and bedded deposits in both the granitic 
and limestone districts. 

It is important then for prospectors to keep a look-out, not 
so much for one particular kind of rock, but the juncture of two 
different rocks, one eruptive and the other not, and prospect at 
the line of juncture. 

The eruptive rock is not always necessary in immediate 
contact with the ore or ore-bearing rock, but may be sufficiently 
near to influence rocks in its vicinity with mineral solutions 
emanating from it. Not a single circumstance but a combination 
of circumstances which we have detailed in this work consti¬ 
tute an ore-bearing area. 

Sometimes a miner may mistake the plane of a fault for a 
true contact; he may have been perhaps for sometime following 
a dipping or blanket vein and encounter suddenly an abrupt 


GEOLOGY OF COLORADO ORE DEPOSLTS. 


CXXXIX 


wall of granite or possibly porphyry, the result of a fault, which 
so far from being a desired contact is practically the terminus 
of his mining operations. Cases have occurred where the miner 
has still pursued the even tenor of his way and gone on 
tunnelling in the granite with, of course, no results. 

We cannot too strongly emphasize what we have said in the 
body of this treatise relative to the popular, mischievous fallacy 
of supposing that richness must almost necessarily increase with 
depth, and that though evidences seem poor for moderate depth 
below the surface, yet “if we only go deep enough we are pretty 
sure to strike it rich.” 

There is no scientific reason one way or the other for this, and 
experience is rather against richness with great depth. Nearly 
all our gold veins are richer and more valuable near the surface, 
and in the oxidized portion than with depth. Oxidized ores 
of all classes within moderate depth from the surface are, as 
a rule, richer and easier treated than those found at great depth. 
Consideration of this fact would prevent much money being 
risked or wasted in expensive, need less cross-cut tunnels in dead 
rock, at some low point, in hopes of tapping the vein at great 
depth. It should also prevent parties unacquainted with mining 
in expecting too much with depth from lodes that have not 
proved very profitable near the surface. Notable and important 
exceptions we admit often occur to this general rule. 

In examining a mining property with a view to forming esti¬ 
mates of its value, a thorough system of assaying should be 
pursued in various parts of the ore body, especially in the more 
average and poorer parts of the vein. Rich specimens should 
be studiously avoided. Assays from such would give a most 
fraudulent report of the mine. There is a certain district in 
Arkansas from which glowing reports have been made, backed 
up by these specimen assays, whilst the region is utterly valueless. 
To show a specimen from a mine coated with native gold or 
silver is a common device of the unscrupulous man who has a 
“hole in the ground” to sell to some “tenderfoot” who is struck 
with mining fever and is too ready to believe what “he sees with 
his own eyes ” By and by “the proof of the pie will come in 
the eating thereof.” 



Appendix. 


THEORIES OF VEIN FORMATION AND ORE DEPOSITS. 

Dr. J. S. Newberry, of Columbia School of Mines, groups 
mineral veins in three categories: i. Gash veins. 2. Segre¬ 
gated veins. 3. Fissure veins. 

Gash Veins. —“Ore deposits confined to a single bed or forma¬ 
tion of limestone , of which the joints, and sometimes planes of 
bedding, enlarged by the solvent power of atmospheric water, 
carrying carbonic acid, and forming crevices, galleries, or caves, 
are lined or filled with ore leached from the surrounding rock; 
e. g., the lead deposits of the Upper Mississippi and Missouri. 

Segregated Veins. —“Sheets of quartzose matter, chiefly 
lenticular, and conforming to the bedding of the enclosing rocks, 
but sometimes filling irregular fractures across such bedding; 
found only in metamorphic rocks, limited in extent laterally and 
vertically, and consisting of material indigenous to the strata in 
which they occur, separated in the process of metamorphism ; 
e. g., quartz ledges carrying gold, copper, iron pyrites, etc., in the 
Alleghanies, New England, Canada, etc. 

Fissure Veins. —“Sheets of metalliferous matter filling fissures, 
caused by subterranean force, usually in the planes of faults, and 
formed by the deposit of various minerals brought from a lower 
level by water, which, under pressure, and at a high temperature , 
having great solvent power, had become loaded with matters 
leached from different rocks, and deposited them in the channels 
of escape as the pressure and temperature were reduced. 

Bedded Veins. —“Are zones or layers of a sedimentary rock, 
to the bedding of which they are conformable; impregnated with 
ore derived from a foreign source, and formed long subsequent to 
the deposition of the containing formation.” Several of the 







cxlii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


mines of Utah are cited, which are all zones in quarztite, which 
have been traversed by mineral solutions that have, by substitu¬ 
tion, converted such layers into ore deposits of considerable 
magnitude and value. 

“The ore contained in these bedded veins' exhibits some 
variety of composition, but where unaffected by atmospheric 
action, consists of argentiferous galena, iron pyrites carrying gold, 
or the sulphides of zinc and copper, containing silver or gold or 
both. The lead carbonate and galena are often stained with 
copper carbonates. In the ‘Green-eyed Monster’ mine of Utah 
the ore, thoroughly oxidized as far as penetrated, forms a sheet 
from twenty to forty feet thick, consisting of rusty, sandy or 
talcose soft material, carrying from twenty to thirty dollars to the 
ton in gold and silver. 

“The quartzites are of Silurian age, but were impregnated by 
metalliferous solutions much later, probably in the Tertiary, and 
after the period of disturbance in which they were elevated and 
metamorphosed. This is proven by the fact that in places where 
the rock has been shattered, strings of ore run off from the main 
body, cross the bedding and fill interstices between the fragments, 
forming a coarse ‘stock work.’ 

“Bedded veins may be distinguished from fissure veins by the 
absence of all traces of a fissure, the want of a banded structure, 
slickensides, gouge, etc.; from ‘gash veins’ and the ‘floors of ore’ 
which accompany them, as well as from segregated veins, they 
are distinguished by the nature of the enclosing rock and the 
foreign origin of the ore. 

Sometimes the plane of juncture between the two contiguous 
sheets of rock has been the channel through which has flowed a 
a metalliferous solution, and the zone where the ore has replaced 
by substitution portions of one or both strata. These are often 
called blanket veins, in the West, but belong rather to the class of 
‘contact deposits.’ 

“Where such sheets of ore occupy, by preference, the planes 
of contact between adjacent strata, but sometimes desert such 
planes and show slickensided walls and banded structure, like the 
great veins of Bingham, Utah, these should be classed as true 
fissure veins.” 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cxliii 


THEORIES OF ORE DEPOSIT. 

Doctor Newberry, as an advocate of the ascension theory, 
differs from Mr. S. F. Emmons and Mr. Becker, who lean 
toward the lateral secretion theory, and who, in their examination 
respectively of the Leadville and Comstock ore deposits, attribute 
the ores to the leaching of adjacent igneous rocks. He differs 
with them for the following reasons : 

“First—The great diversity of character exhibited by different 
sets of fissure veins which cut the same country rock seems 
incompatible with any theory of lateral secretion. 

“These distinct systems are of different ages, of diversified 
composition, and have drawn their supply of material from 
different sources. For examples, the Humboldt, the Bassick and 
Bull Domingo, near Rosita and Silver Cliff, Colorado. These 
are veins contained in the same sheet of eruptive rock, but the 
ores are as different as possible. The Humboldt is a narrow 
fissure, carrying a thin ore streak of high grade, consisting of 
sulphides of silver, antimony, arsenic and copper. The Bassick 
is a great conglomerate vein, containing tellurides of silver and 
gold, argentiferous galena, blende and yellow copper pyrites. 
The Bull Domingo is also a great fissure, filled with rubbish, 
containing ore chimneys of galena, with tufts of wire silver.” 
Many other groups of mines are also cited, showing that the 
same rocks are cut by veins of different ages, having different 
bearings and containing different ores and veinstones. “It seems 
impossible that all these diversified materials should have been 
derived from the same source, and the only explanation is the 
ascent of metalliferous solutions from different and deep seated 
sources. 

“These and all similar veins have certainly been filled with 
material brought from a distance and not derived from the walls!' 

LEACHING OF IGNEOUS ROCKS. 

Against the theory that mineral veins have been produced by 
the leaching of superficial igneous rocks, he says: 

“Thousands of mineral veins the world over occur in regions 
remote from eruptive rocks,” and cites a great number of 




cxliv GEOLOGY OF COL OF A BO OFF DEPOSITS. 

examples. “In the great mineral belt of the Far West, where 
volcanic emanations are so abundant, and where they have 
certainly played an important part in the formation of ore 
deposits, the great majority of veins are not in immediate 
contact with trap rocks, and they could not, therefore, have 
furnished the ores.” He cites several examples, and amongst 
them “the gold mines of Black Hawk, Colorado, the Montezuma, 
Georgetown, and other silver mines in the granite belt of 
Colorado. In nearly all the localities cited we may find evidence 
of not only that the ore deposits have not been derived from the 
leaching of igneous rocks, but also that they have not come from 
those of any kind which form the walls of the veins. 

“The gold bearing quartz veins of Deadwood, (Black Hills) 
Dakotah, are so closely associated with dikes of porphyry that 
they may have been considered as illustrations of the potency of 
trap dikes in producing concentration of metals. But we have 
evidence that the gold was there in Archcean times, while the 
igneous rocks are all of modern, probably of Tertiary date. This 
is shown in ‘the cement mines’ of the Potsdam Silurian sand¬ 
stone. This is the beach of the lower Silurian sea when it 
washed the shores of an Archoean island, now the Black Hills. 
The waves that produced this beach beat against cliffs of granite 
and slate, containing quartz veins carrying gold. Fragments of 
this auriferous quartz and the gold beaten out of them and 
concentrated by the waves, were in places buried in the sand 
beach in such quantity as to form deposits, from which a large 
amount of gold is now being taken. Without this demonstration 
of the origin and antiquity of the gold it might very well have 
been supposed to be derived from the eruptive rock.” 

Again he says : “That where igneous rocks are most preva¬ 
lent such districts are proverbially barren of precious metals, and 
where these metals do occur in such districts the same sheet of 
rock may contain several systems of veins with different ores and 
gangues.” He cites the great lava plains of Snake River, of 
Eastern Oregon, Northern California and New Mexico as unpro¬ 
ductive generally of ore deposits. Also the great lava plateaus 
of the Cascade range. On the other hand the Sierra Nevada, 
composed principally of melamorphic rocks, contains vast quan¬ 
tities of gold, silver and copper. 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cxlv 


“At Lake City, in the San Juan district of Colorado, the 
prevailing porphyry holds the veins of the ‘Ute’ and ‘Ulay’ and 
‘Ocean Wave’ mines, which are similar, whilst the ‘Hotchkiss’ 
and ‘Belle’ are entirely different. We have no evidence that any 
volcanic eruption has drawn its material from zones or magmas 
especially rich in metals or their ores. And on the contrary 
volcanic districts like those mentioned, and regions such as the 
Sandwich Islands, where the greatest eruptions have taken place, 
are poorest in metalliferous deposits.” 

He remarks that igneous rocks in our Western territories are 
“but fused conditions of sediments forming the underlying 
structure of that country.” [They may be fused archoean 
granitic rocks, but certainly not the higher series of silurian, 
carboniferous and palceozoic rocks, for the simple reason that at 
Leadville and in our western region we can readily trace the » 
vents or dykes from which this eruptive rock came up, pene¬ 
trating deep into and lost in unknown depths of the granite, and 
thence rising and spreading over and intruded into the overlying 
Paleozoic rocks, as its source is evidently far beneath these 
overlying rocks, its material could not have been derived from 
them.] 

“Over the great mineral belt which lies between the Sierra 
Nevada and the front range of the Rocky Mountains, and 
extends not only across the whole breadth of that region, but 
far into Mexico, the surface was once underlain by a series of 
Paleozoic sedimentary strata, not less than twenty thousand feet 
in thickness, and beneath this were Archoean rocks, also metamor¬ 
phosed sediments. Through these the ores of the metals were 
generally, though sparsely, diffused. In the convulsions which 
have in recent times broken up this long quiet and stable portion 
of the earth’s crust, (and which have resulted in depositing in 
thousands of cracks and cavities the ores we now mine) portions 
of the old table land were in places set up at high angles, forming 
mountain chains and doubtless extending to the zone of fusion 
below. Between these blocks of sedimentary rocks oozed up 
through the lines of fracture, quantities of fused material, which 
also sometimes formed mountain chains, and it is possible and 
probable that the rocks composing the volcanic ridges are but 


cxlvi 


GEOLOGY OF COLORADO ORE DEPOSITS. 


phases of the same materials that form the sedimentary chains.” 
[Of the granite, perhaps, but not of the limestones.] “There is 
no particular reason why the leaching of one group should 
furnish more ore than the other, but as a fact the unfused sedi¬ 
ments are much the richer in ore deposits. This is to be 
accounted for by supposing that they have been the rceptacles of 
ore brought from a foreign source. We conclude that there has 
been a zone of solution below, where steam and hot water, under 
great pressure, have effected the leaching of ore bearing strata, 
and a zone of deposition above, where cavities in preexistent, solid¬ 
ified, and shattered rocks became the repositories of the deposits 
made from ascending solutions when the temperature and 
pressure were diminished. Where great masses of hot lava were 
poured out, these for a long time remained too heated for ore 
deposition. So long, indeed, that the period of active vein 
formation may have passed before they reached a degree of solidi¬ 
fication and coolness that would permit their becoming receptacles 
of the products of deposition. The masses of unfused cooler 
sedimentary rocks forming the most metalliferous mountains 
were, throughout the period of disturbance, in a condition to 
become such repositories. Highly heated solutions, forced by an 
irresistable power through rocks of any kind down in the heated 
zone, would be far more effective leaching agents than cold 
surface water, with feeble solvent power, moved only by gravity, 
percolating slowly through superficial strata. 

“Richtofen suggested that the mineral impregnation of the 
Comstock lode was the result of the leaching of deep seated 
rocks, perhaps the same that enclose the vein above, by highly 
heated solutions which deposited their load near the surface. 

“Becker supposes the ore concentration to have been effected 
by surface waters flowing laterally through the igneous rocks, 
gathering the precious metals and depositing them in the fissure, 
as lateral secretion produces the accumulation of ore in the lime¬ 
stone of the lead region.” 

Prof. Newberry thinks Richtofen’s theory the most probable 
of the two. 

“For, first, the veinstone of the Comstock is chiefly quartz, 
the natural, common precipitant of hot waters, since they are far 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cxlvii 


more powerful solvents of silica than cold waters. The ores 
deposited in the Mississippi lead region at low temperature 
contain little silica. 

“Second—The great mineral belt alluded to between the 
Rockies and Sierra Nevada is now the region where nearly all 
the hot springs of the continent are situated. It is evident that 
these are the last of the series of thermal phenomena connected 
with the great volcanic upheavels and eruptions, of which this 
region has been the theatre since the beginning of the Tertiary 
age, and it is evident that the number of hot springs in this 
region was once far greater even than it is now. That these hot 
springs were capable of producing mineral veins by materials 
brought up in and deposited from these waters, is demonstrated 
by the phenomena of the Steamboat Springs, where we have the 
best illustration of vein formation now in progress. The temper- * 
ature of the lower workings of the Comstock mine is now over 
150° F., and an enormous quantity of hot water is discharged 
through the Sutro tunnel. This water has been heated by 
coming in contact with hot rocks at a lower level than the present 
workings of the Comstock lode, and has been driven upward in 
the same way that the flow of all hot springs is produced. As 
that flow is continuous it is evident that the workings of the 
Comstock have simply opened the conduits of hot springs, which 
are doing to-day what they have been doing in ages past, but 
much less actively, that is, bringing toward the surface the 
materials they have taken into solution in a more highly heated 
zone below. Hence it seems more natural to suppose that the 
great sheets of ore bearing quartz now contained in the Comstock 
fissure were deposited by ascending currents of hot alkaline 
waters than by descending currents of those which were cold and 
neutral. The hot springs are there, though less copious and less 
hot than formerly, and the natural deposits from hot water are 
there. It seems more rational to suppose, with Richtofen, that 
these are related as cause and effect rather than that cold water 
has leached the ore and the silica from the walls near the surface. 
The fissure was for a long time filled with a hot solution charged 
with an unusual quantity of the precious metals, and the 
presence of gold in the wall rock is due to their being partly 
impregnated with the same solution. 


cxlviii GEOLOGY OF COLORADO ORE DEPOSITS. 


“At Leadville there are no facts to prove that the ore deposits 
have been formed by the leaching of the overlying porphyry 
rather than by an outflow of heated mineral solutions along the 
plane of junction between the porphyry and the limestone. Near 
this plane the porphyry is often thoroughly decomposed, is some¬ 
what impregnated with ore, and even contains sheets of ore 
within itself; but remote from the plane of contact with the 
limestone it contains little diffused and no concentrated ore. It 
is scarcely more pervious than the underlying limestones, and 
why a solution that could penetrate and leach ores from it should 
be stopped at the upper surface of the blue limestone is not 
obvious. Nor why the plane of junction between the porphyry 
and the blue limestone should be the special place for the deposit 
of the ore.” 

In place of Mr. Emmons’ theory of the leaching of the 
porphyry by waters from above, etc., Prof. Newberry thinks that 
the Leadville ore deposits “can be better accounted for by 
supposing that the plane of contact between the limestone and 
porphyry has been the conduit through which heated mineral 
solutions, coming from deep seated and remote sources, have 
flowed, removing something from both the overlying and under¬ 
lying strata, and, by substitution, depositing sulphides of lead, 
iron, silver, etc., with silica.” 

If the porphyry is so rich in precious metal as Mr. Emmons’ 
assays report it to be, Prof. Newberry thinks it “a remarkable and 
exceptional case of the diffusion of silver and lead through 
igneous rocks.” He thinks it possible that the Leadville 
porphyries are phases of rock rich in silver, lead and iron, which 
underlie this region, and which have been fused and forced to the 
surface by an ascending mass of deeper seated igneous rock ; “but 
even if the argentiferous character of the porphyry shall be 
proven, it will not be proven that such portions of it as here lie 
upon the limestone have furnished the ore by the descending 
percolation of cold surface waters. Deeper lying masses of this 
same silver, lead and iron bearing rock, digested in and leached 
by hot waters and steam, under great pressure, would seem to be 
a more likely source of the ore.” He argues also that if the 
overlying porphyry had yielded, by leaching, such enormous 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cxlix 


bodies of metal as we find in the Leadville ore bodies, we should 
find the porphyry a much more rotten, “digested kaolinized and 
desilicated rock than it is. As a rule it is generally quite compact, 
except in a narrow line near the ore body, where it is much 
decomposed, probably by hot chemical solutions forced up from 
below, along the plane, of contact. It is difficult to understand 
why the upper portion of the porphyry should be so solid and 
homogeneous, with no local concentrations or pockets of ore, 
if they have been exposed to the same agencies as those which 
have so changed the under surface.” 

He thinks also that if the ore bodies were derived by leaching 
of the porphyry by surface waters, there ought to be evidence of 
its continuing in the present day as it does in some galena mines. 
Dr. Newberry admits, in a modified form, the general truth of a 
“lateral secretion” theory in some instances, but does not think 
that the vein materials are necessarily derived from the wall rock 
immediately opposite the places where the ore is found. He 
considers the main influence of igneous rocks is rather in 
supplying heat to the solutions than directly supplying the 
elements of the ore, and he mentions a case in Utah where ore 
bearing quartz veins come up on either side of an unaltered, hard 
dyke of igneous rock, from which there is no evidence that they 
derived either their quartz or their metal from the dyke or adja¬ 
cent limestone, but from heated solutions ascending from a deeper 
source and bringing up foreign material with it. He considers 
Richtofen’s theory of the filling of the Comstock to be the true 
one, and the phenomena furnished by Steamboat Springs to give 
us the typical mode in which most metallic veins were formed 
and filled. 

LE CONTE’S THEORY. 

Professor Le Conte, in his geology, says of fissure veins, that 
they are fillings of great fissures, produced by movements of the 
earth’s crust. When these fissures are filled, at the time of 
formation by igneous injection, they are called dykes, when subse¬ 
quently with mineral matter by a different and slower process 
they are fissure veins. They often outcrop like dykes for miles 
over the surface of the country by reason of their superi or 
ness to the enclosing country rock, and extend to unk 


cl 


GEOLOGY OF COLORADO ORE DEPOSITS. 


certainly very great depths. They also occur in parallel systems. 

The leading characteristics of true fissure veins he defines 
to be : 

1. Their continuity for great distances and to great depths. 

2. Their occurring in parallel systems. 

3. Their filling a preexistent fissure, the distinction between 
vein and wall rock being usually quite marked. 

4. The presence of selvage or gouge between the gangue 
and the country rock, which he attributes to water circulating in 
the fissure. 

5. Their contents are more varied than those of other classes 
of ore deposits. 

He distinguishes infiltration veins and great fissure veins, the 
former occupying a small short crack in the strata, and deriving 
its filling from material oozing from the sides by lateral secretion 
from a single variety of rock. The latter occupy great and deep 
fissures, and derive their contents from all the strata to great 
depths, and especially from the deeper strata. Hence the contents 
of these veins are more varied. 

“The contents of mineral veins were deposited by hot alkaline 
solutions coming up through fissures, in other words hot alkaline 
springs. Deposition by solution is proved by the occurrence ot 
banded or ribbon structure and interlocked crystals and combs, 
by quartz forming the gangue, and that quartz of the kind known 
to be only formed by water solutions by its containing bubbles 
of water inside it, etc. 

“That the solutions were hot is implied by the great depth to 
which the fissures are known to descend and the regular increase 
with depth, of the heat of the earth. Hot water is also a most 
powerful solvent. That the solutions were alkaline is implied by 
the fact that alkaline sulphides and carbonates are the only 
solvents of quartz. The same character of water when carbonic 
acid is in excess, dissolves carbonate of lime and baryta, the 
other common forms of gangue. Hot springs of this kind in 
Nevada are to-day depositing quartz and iron and lime and 
filling fissures.” 

He considers that the ore or metal materials came in with the 
same solutions that brought in the dissolved quartz, lime or 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cli 


baryta gangue. He considers that great fissures have been 
formed by deposit from hot alkaline waters holding various 
mineral substances in solution. The more insoluble substances 
are deposited in the vein, while the more soluble reach the 
surface as mineral springs, and he quotes the already described 
phenomena of the Steamboat Springs, near Virginia City, 
Nevada, as a fissure vein forming before our eyes, and explaining 
to our vision the way in which fissure veins have in former ages 
probably been formed. 

He sums up by saying that: “Meteoric waters circulating in 
the interior of the earth in any direction, downward, upward, or 
laterally, deposit slightly soluble matters in their course, in 
cracks, cavities or great fissures, forming fossil casts, geodes, 
amygdules, infiltration veins and fissure veins. 

“As to direction—the up-coming waters, especially in meta- 
morphic and volcanic regions, deposit most freely, because 1 they 
are hot and alkaline, and therefore powerful solvents and cool 
gradually on approaching the surface. But that downward perco¬ 
lating waters may also deposit metallic ores is proved by the fact 
that these are sometimes found hanging like stalactites from the 
roof of cavities. The great fissure veins are the most prolific 
because these fissures are the highways of water from the heated 
depths. But every kind of water-way will receive deposits, and 
as the kinds of these are infinitely various, and pass by insensible 
gradations into each other, so also will be the veins that fill them. 
The open fissure is the easiest, and therefore the most traveled 
highway. In these, therefore, we have the most perfect type of 
veins, with their banded structure, their selvages (or gouge), their 
great size and continuity. But in many cases crust movements 
produce only slight fissures or loosening of the rocks along 
planes full of small cracks with country rock between. These 
loosened planes become also water-ways, and, by deposit, form 
those irregular veins so common everywhere, but especially in 
the cinnabar veins of California 

“Or again, crust movements may produce not clean, open 
fissures, but rather planes of shattered rock like fissures filled 
with rubble. Deposits in such a water-way forms a breccia of 
country rock, cemented with vein stuff. 


V 


clii GEOLOGY OF COLORADO ORE DEPOSITS. 

“Or again, in certain country rocks, soluble in water, especially 
limestones, the rock is dissolved along the water-way and the 
vein stuff deposited ‘pari passu,’ giving us the substitution veins. 
In short, if one can conceive clearly that mineral veins are filled 
water-ways, all these complex phenomena solve themselves.” 

DESCRIPTION OF FRONTISPIECE. 

The frontispiece represents both sides of the Roaring Fork 
Valley in which Aspen city is situated, and also looks up 
toward the Grand Canon of Roaring Fork, where the river 
issues through its walls of granite in the Sawatch Range. On 
the right hand side of the picture we see Aspen Mountain on 
which are so many of the principal mines. These are located 
principally on the slope of Spar Gulch and in the bed of the 
adjacent Vallejo Gulch. In the Spar Gulch are the Durant and 
other “Apex” mines, and in Vallejo are the principal “Sideline” 
mines, both working in the same uplifted belt of carboniferous 
limestone and both, happily, of late consolidated by a compromise. 
The centre of Vallejo Gulch is occupied by a great thickness of 
eruptive diorite lying more or less upon top of the basined 
and faulted limestone, and separated from it by a belt of black 
shale. On the opposite side of this, on the right, the limestone 
again appears by a process of folding and faulting, and is pierced 
by several tunnels, amongst them the Late Acquisition and 
Pioneer Mines. The limestone and some quartzite rest upon 
granite which forms the central nucleus or basis of the mountain 
which, as the engraving shows, is hollowed out into a thin she’ll 
like the half-section of the crater of a volcano. On the north¬ 
western slope of this shell are seen the same silurian quartzites and 
overlying carboniferous limestones leaning against and folding 
around the granite with a somewhat different direction of strike 
and dip from the beds on the opposite side of the shell in Spar 
Gulch. Ore deposits are found on this side also, but not so 
abundantly as on the other; the Pride of Aspen, Mary B and 
Homestake tunnels have found ore. Beneath this slope is Castle 
Creek, flowing into the Roaring Fork. This side of the Roaring 
Fork, together with the city of Aspen as it appeared two years 
ago when the writer sketched it, together with the principal 


GEOLOGY OF COLORADO ORE DEPOSITS. 


cliii 


mines on Aspen Mountain are best shown in the sketch on page 
cxx, with the accompanying section. On the left hand of the front¬ 
ispiece, the small but important hill called Smuggler Mountain is 
seen. Its importance consists in the rich developments of ore 
that have been found in the Smuggler, J. C. Johnson and Regent 
mines, all of them, as on> the opposite mountain, apparently on 
the same line of contact. The geology of this mountain is some¬ 
what obscured by enormous coverings of glacial drift, but from 
the developments in the mines it appears to correspond to the 
same formations as are more clearly developed on the opposite 
Aspen Mountain in Spar Gulch. The mountain is of less size 
than that of Aspen Mountain apparently from the strata not 
being there reduplicated by faulting, as on Aspen Mountain, but 
formed of a single series of strata consisting of silurian 
quartzites resting on granite, with dolomitic limestone (short 
limestone) and black shales above them, and also some diorite 
porphyry (if we may judge from materials found in the dumps) 
included, as on the other side, in the black shales. Though the 
ore bodies are well developed in the mines and of much the same 
character as on Aspen Mountain, the presence of the blue lime¬ 
stone of Spar Gulch is not so apparent; its seeming absence 
might be accounted for by its having been more completely 
dolomitized on this side than on the other, or by its having been 
partially or completely eroded away or even replaced by ore, 
rather than to suppose the strata is different or the ore belt at a 
different level on this side than on the other. It is probable that 
a fault corresponding with the bed of Roaring Fork will be found 
to separate this mountain from Aspen Mountain. The dip of the 
strata, as shown in the J. C. Johnson mine, is much steeper than 
that on Aspen Mountain, and increases in steepness with depth. 
The strata on the opposite side of Hunter’s Creek are red sand¬ 
stones, and probably belonging to the Jura-Trias, or extreme upper 
Carboniferous. In the foreground is a pretty little artificial lake, 
a favorite resort of the Aspenites. Behind the lake appear some 
of the houses on the skirts of the city. 

SAN JUAN FISSURE VEINS. (Page J2.) 

The illustrations, Figs, i and 2, representing “horses” in great 
fissure veins, were from sketches by the writer, of some fissure 


X 


cliv GEOLOGY OF COLORADO ORE DEPOSITS. 

veins in the Animas Canon, between Silverton and Animas 
Forks. The veins are of great width, from 50 to 100 feet in 
places, and are clearly defined on the side of the cliffs of the 
canon, whose height is between 2,000 and 3,000 feet. Fig. 3 is 
from Dr. Hayden’s report, of two series of fissure veins cutting 
one another diagonally, opposite Howardsville, in the same 
canon. Two mammoth master veins are cut by a series of minor 
fissure veins having a different direction or strike. 

SHEEP MOUNTAIN FOLD AND LONDON FAULT. (Page 33.) 

The picture of the “Sheep Mountain fold and London fault” 
is a striking example of structural geology, showing how a moun¬ 
tain range, such as the Mosquito Range, is formed by a series of 
folds, passing at their greatest tension into profound fractures or 
faults, resulting from tangential pressure or compression, whose 
ultimate cause, is the gradual contraction of the earth’s crust 
around its diminishing cooling nucleus. 

It is not often that nature supplies us with so remarkably 
clear a section of her hidden structure as that presented in the 
Sheep Mountain fold in Horseshoe Gulch. The hard Silurian 
quartzites, together with the overlying Carboniferous sandstones, 
shales and limestones, and a cap of white porphyry, are seen 
bending over in a steep but complete arch almost as perfect to 
the eye as one formed of artificial masonry. 

The exact line of the London fault, which is indicated by the 
little valley or sag between it and the adjoining Lamb Mountain, 
is, as is usual in the case of faults, obscured by debris and vegeta¬ 
tion. If the debris were removed we should probably find the 
fold passing into an S-like form, broken near the base by a nearly 
vertical crack, with Silurian strata and Archaean granite forming 
the east wall of the crack abutting against upper Carboniferous 
grits on the west wall of the fissure. There is consequently a 
slip here of many hundreds of feet. Some idea of the amount of 
slip may be obtained by noticing that the Weber grits of the 
upper Carboniferous, marked g, lie properly on top of Sheep 
Mountain. The fault has dropped them down to the bottom of 
the valley, where the upper portion of them is seen outcropping 
at the base of the adjacent Lamb Mountain. A portion of them, 


GEOLOGY OF COLORADO ORE DEPOSITS. 


civ 


turned up by the fault at a sharp angle, is observable on the east 
side of Lamb Mountain, reclining against eruptive white porphyry. 
In the grits and shales were found well defined impressions of 
true Carboniferous foliage, such as the Equiseta. The top of 
Lamb Mountain gives us a good example of a laccolite of 
eruptive white poryhyry,. from which the overlying and once 
over-arching sedimentary strata have been removed by erosion. 
A dyke of white poryhyry is found cutting through the under¬ 
lying strata and connecting with the great laccolitic mass, which 
rests intrusively on the Weber grits. This is one of the numerous 
vents from which the great white porphyry sheets came. 

SUMMIT OF MX. LINCOLN. (Page 49.) 

Mt. Lincoln is a typical example of a mountain formed by a 
net work of dykes and branching sheets of hard porphyry 
intruded between less enduring strata of silurian quartzites and 
carboniferous limestones, welding together the mass into a 
compact form which has resisted erosion and left a prominent 
peak 14,000 feet above sea level and 4,000 feet above the 
adjacent valleys of erosion. A dyke of coarse grained quartz- 
porphyry, containing large perfect crystals of feldspar, has come 
up through the granite, piercing also the overlying paloeozoic 
strata, sending out intrusive sheets, and near the top intruding an 
enormous thick reservoir mass between the strata, in the form of 
a laccolite, from which the overlying and once over-arching strata 
has been removed, leaving the great columnar mass exposed, 
which now forms the summit of the peak. This dyke has also 
been cut by newer dykes and sheets of a dark green porpyhry 
called porphyrite, which has also sent out horizontal sheets 
between the silurian strata, and these sheets have again been cut 
by a dyke of white porphyry. The feeding dykes of both forms 
of porphyry are here traceable down into the granite forming the 
base of the mountain. The principal ore belt, upon which the 
Russia, Present Help and other mines are located, is near the 
summit of the mountain, at the contact of a porphyry sheet with 
the carboniferous limestone, The Present Help mine is one of 
the highest in the world. 


clvi 


GEOLOGY OF COLORADO ORE DEPOSITS. 


THE COLORED GENERALIZED SECTION OF THE ROCKY MOUNTAINS IN 
COLORADO, SHOWING THE POSITION OF ECONOMIC PRODUCTS 
IN THE DIFFERENT GEOLOGICAL HORIZONS AND STRATA. 

(Page cxxviii or 128.) 

This section, which illustrates portions of Part I. and II. of 
this treatise, represents a general average section of the Rocky 
Mountains in Colorado, made up from typical sections of the 
country where the strata are best exposed. 

Thus the Archaean may be exemplified by the Sawatch and 
portions of the Mosquito ranges. 

■ The Paloeozoic, consisting of the Silurian and Carboniferous, 
by the Leadville, South Park and Aspen districts. 

The Triassic and Jurasssic, by the rocks of Morrison or by 
the Garden of the Gods at Manitou. 

The Lower Cretaceous, to the Laramie coal beds, also by the 
section near Morrison, along the banks of Bear Creek. 

The Laramie Cretaceous, including the coal beds, by the 
section at Golden City, along the banks of Clear Creek. 

The Tertiary, by the table lands or “mesas” of the Divide 
near Sedalia. 

The Quaternary and Tertiary, by the strata underlying 
Denver and forming the Denver basin. The Quaternary pebble 
beds are distributed at intervals over the eroded tops of all the 
formations from Archaean to Tertiary, and from the high moun¬ 
tain placers in glaciated canons to where the principal streams 
debouche on the prairies. 

The Archcean is shown to consist of granite, gneiss, schist, 
etc., traversed by dykes of eruptive rock and by fissure veins 
carrying silver, gold, lead and iron, and constitutes the axis of 
the high mountain region. 

The Silurian and Carboniferous, constituting the Paloeozoic 
era, rest on the granite, and are traversed by various eruptive 
porphyries issuing from the underlying Archaean, in dykes, and 
spreading out between the Paloeozoic strata in intrusive sheets, 
or in thick laccolitic masses, sometimes by erosion, forming the 
caps or peaks of prominent mountains. The quartzites are shown 
to be principally gold bearing and the limestones silver and lead 
bearing, particularly at their contact with eruptive porphyries. 


GEOLOGY OF COLORADO ORE DEPOSITS. clvii 

Iron is found throughout the series. The great thickness of 
Weber grits, consisting of coarse sandstones, shales and quartzites, 
with a few limestones, are generally unproductive. A few thin 
seams of semi-anthracite coal occur at Aspen and Leadville, and 
towards the upper portion some limestones, penetrated by 
porphyries, produce important deposits of silver and lead in the 
Ten-Mile district at Kokomo, of which the Robinson mine may 
be taken as typical. 

The Paloeozoic rocks are generally confined to the high 
mountain region. 

The Mesozoic rocks, consisting of the Trias, Jurassic and Creta¬ 
ceous, are more characteristic of our foothills and hogbacks, and 
yield us no precious metals, but many valuable economic products. 

The Trias yields good red building stone at the Glencoe 
quarries, on Ralston Creek ; also white silicious sandstone for 
glass manufacture, and red flagging stone in Boulder County. 

The Jurassic yields quicklime from quarries in limestone, at 
several points along the foothills; also beds of gypsum for 
plaster of paris, and some fine red building sandstone. The 
formation is remarkable for the dinosaurs, discovered at 
Morrison and Canon City, and for the first discovery of oil on 
Oil Creek, near Canon City. 

The base of the Cretaceous, called the Dakotah group, 
consists of a thick bed of white sandstones, quarried at Morrison 
for building stone, and in the center of the group is a belt of the 
finest blue fireclay in America, quarried between Golden and 
Morrison, and extensively used for fire brick, etc. This formation 
generally forms a prominent hogback along our foothills. 

The Colorado Cretaceous consists first, of a bed of black 
shale, with concretions of inferior iron ore; secondly, of white 
limestone, much quarried along the foothills at Golden, Morrison, 
Canon and elsewhere for flux for the smelters ; thirdly, a bed, 
sometimes over 2,000 feet thick, of drab shales and clays, near 
the middle of which is a sandy layer, which has been tapped by 
the oil wells of Florence, near Canon City, and has so far yielded 
most of our oil production. 

The Laramie Cretaceous has a thick bed of white sandstones 
near its base, which is utilized for building sandstone, and 


clviii 


GEOLOGY OF COLORADO ORE DEPOSITS. 


quarried at Trinidad, Canon City and elsewhere. In this sand¬ 
stone formation lie the principal coal seams, the largest and 
most worked averaging 6 to 8 feet in thickness. Beds of inferior 
concretionary iron ore are also found near the coal, and also beds 
of plastic clay and inferior fire clay. 

The Tertiary beds of clays and sandstone, underlying Denver, 
have given us our artesian wells, at depths varying from 300 to 
1,000 feet, and on the Divide, between Denver and Colorado 
Springs, the Tertiary table lands are capped by a pink rhyolite 
lava, much used as building stone. 

The Quaternary , consisting of drift, pebbles and clay, can be 
found on the banks of our streams and underlying our farm 
lands, or on the sides of the deep canons on the high mountain 
areas. It forms our placer beds, which yield more or less gold, 
and in several localities the fine clay makes good red building 
brick. Above this rests the black soil of the farm lands, with 
their crops of grain, grass and vegetables. 

This section can be applied, with modifications, in a general 
way, to various parts of Colorado, and may act as a rough 
guide or map to the prospector in search of precious metals or 
other economic products. 

SECTIONS ILLUSTRATING THE GRADUAL DEVELOPMENT OF THE 

LEADVILLE AND SOUTH PARK REGION. (Page Cxii Or I 1 2.) 

Figures i and 2 are ideal, and meant merely as illustrations 
of Mr. Emmons hypothesis, with a view to make the text more 
clearly understood. 

Fig. I represents all the strata, from Silurian to Cretaceous, as 
lying conformably on one another at the bottom of the Cretaceous 
sea, between two granite islands—the one on the right constituting 
the “nucleus” of the modern Front Range, the one on the left of 
the Sawatch Range. Between the two is the area now occupied 
by the South Park, the Mosquito Range, the Leadville district 
and the Arkansas Valley. Ihese conformable strata were, 
according to Emmons, penetrated by dykes and intrusive eruptive 
sheets whilst lying below the sea, and mineralization took place 
about the same time. 


GEOLOGY OF COL OF A DO ORE DEPOSITS. 


clix 


Fig. 2 shows all these strata, with their included eruptive 
sheets and ore beds, crumpled up between the Front Range and 
the Sawatch Range into the Mosquito Range and South Park 
basin, which took place at the great mountain uplift at the close 
of the Cretaceous. 

Fig. 3 is an actual average section of the region of South 
Park and the Leadville district as it now is, showing how the 
folds passed into faults, and how the tops of the folds and uplifted 
cliffs of the faults have been planed down by erosion. 

The figures also show in somewhat the same way the 
progressive history of the eastern and western foothills on either 
side of the granite axes ; how they were originally horizontal, 
how they were folded up and how, by erosion, they have been 
cut down into low hogbacked ridges. 



INDEX OF CONTENTS. 


Part I. 

General Geology. 5— 12 

General Geology of Colorado.•. 12— 16 

Geological age of Colorado ore deposits. 16 

Connection between mineral belts and mountain uplifts_ 17— 18 

Ore bearing rocks. 19 

Part II. 

Mode of occurrence of Colorado ores. 21 

Placers. 23— 28 

Iron ore. 28— 32 

Ore beds. 33 

Faults... 33— 36 

Fissure veins. 38— 48 

Eruptive igneous rocks.. 48— 54 

Other ore bearing rocks. 54— 58 

Characteristics and structure of veins. 59— 64 

Ores, high and low grade, and common varieties described... 64— 70 

Breccias, Horses, Ribbon structure, etc. 71— 74 

Influence of country rock. 74— 77 

Palceontology of ore deposits and generalized economic section 

of Colorado strata. 78— 82 

Theories of origin of ore deposits.84—96 and 141—151 

Part III. 

Colorado mining districts. 98—139 

Boulder county. 98—102 

Gilpin couuty.102—103 

Clear creek county.103—104 

Summit county.104—106 

Park county.107—109 

Lake county, Leadville and South Park district.109—117 

Gunnison county.117—119 

Pitkin county and Aspen mining region.119—131 





























INDEX OF CONTENTS. 


San Juan region.131 13 J 

Custer county and Silver Cliff district.134 -135 

Huerfano county and Spanish peaks. 135 

Conclusion, General remarks.136—139 


Appendix. 


Dr. Newberry and Prof. J.Le Contes’ theories of ore deposits 141 —151 
Description of plates; ..152—159 


Index op illustrations. 


Frontispiece. Aspen looking up Roaring Fork. 

Sheep mountain fold and London fault, South Park. 

Summit of Mt. Lincoln, South Park. 

Fissure veins in San Juan. 

Generalized economic section of Rocky Mts. of Colorado. 

Sections illustrating Geological development of Leadville and 

South Park region. 

Aspen mountain and city from Smuggler hill. 

Section of Aspen mountain. 


1 

33 y 


48 

72 

80 


/ 


112 

120 

128 



LB My ’05 


















/ 












/ 





































